user13147: This board shows the black pieces on the bottom, but the white pieces are supposed to be there; I accidentally set up the board so that white's pieces are on the black side. I am looking to flip the board with the following FEN:
# R2K1B1R/PPP2QPP/5N1N/1n2q3/8/1p3n2/pbpp3p/1kr4r w KQkq - 0 1

Board:
R2K1B1R/PPP2QPP/5N1N/1n2q3/8/1p3n2/pbpp3p/1kr4r w KQkq - 0 1

How may I accomplish this without re-setting the pieces manually? I tried switching the rows, but I got:
# 1kr4r/pbpp3p/1p3n2/8/1n2q3/5N1N/PPP2QPP/R2K1B1R w KQkq - 0 1

Board:
1kr4r/pbpp3p/1p3n2/8/1n2q3/5N1N/PPP2QPP/R2K1B1R w KQkq - 0 1


user14595: The free chess GUI Arena does what you are asking and many other useful things for creating or studying chess problems.
It works with epd files, fen strings and pgn databases.

user16593: I disagree with Jossie's solution.  If you follow that method, and flip the initial game board, you'd end up with queens on the wrong colors.  Doing a vertical flip and switching the colors is the method I use.  
Detailed efficient methods of performing these board flips (as well as rotations) can be found here.
UPDATE:
As Jossie has since pointed out, his solution is correct for the question asked.  My solution is for when you need to transform a board (oriented with white at the bottom and black at the top) in which it's black's turn, to one in the same orientation, but seen as if it was white's turn.  An example of when you'd use my solution is converting a database of boards to one where they're all being represented with the same orientation, and same player to move.

user13147: Although one may try to place the first row as the eighth, the second row as the seventh, etc., the board will appear flipped (as shown above).
This is a horizontal flip (mirror-image over the x-axis) and does not look quite right. Switching the letters from lower-case to capital and vice-versa would switch the colors, but that's not the best solution. 
Upon closer examination, a vertical flip (or mirror over the y-axis) of this horizontal flip would solve the problem: Reverse each sub-string delimited by a slash until you hit the second field, then leave it as is.
The final product is the same FEN, except the first field is a reverse string of itself.
# r4rk1/p3ppbp/2n3p1/8/3q2n1/N1N5/PPQ2PPP/R1B1K2R w KQkq - 0 1

Board:
r4rk1/p3ppbp/2n3p1/8/3q2n1/N1N5/PPQ2PPP/R1B1K2R w KQkq - 0 1


13147: <p>This board shows the black pieces on the bottom, but the white pieces are supposed to be there; I accidentally set up the board so that white's pieces are on the black side. I am looking to flip the board with the following FEN:</p>

<pre><code># R2K1B1R/PPP2QPP/5N1N/1n2q3/8/1p3n2/pbpp3p/1kr4r w KQkq - 0 1
</code></pre>

<p>Board:</p>

<pre><code>R2K1B1R/PPP2QPP/5N1N/1n2q3/8/1p3n2/pbpp3p/1kr4r w KQkq - 0 1
</code></pre>

<p>How may I accomplish this without re-setting the pieces manually? I tried switching the rows, but I got:</p>

<pre><code># 1kr4r/pbpp3p/1p3n2/8/1n2q3/5N1N/PPP2QPP/R2K1B1R w KQkq - 0 1
</code></pre>

<p>Board:</p>

<pre><code>1kr4r/pbpp3p/1p3n2/8/1n2q3/5N1N/PPP2QPP/R2K1B1R w KQkq - 0 1
</code></pre>
 14595: <p>The free chess GUI Arena does what you are asking and many other useful things for creating or studying chess problems.</p>

<p>It works with epd files, fen strings and pgn databases.</p>
 16593: <p>I disagree with Jossie's solution.  If you follow that method, and flip the initial game board, you'd end up with queens on the wrong colors.  Doing a vertical flip and switching the colors is the method I use.  </p>

<p>Detailed efficient methods of performing these board flips (as well as rotations) can be found <a href="https://chessprogramming.wikispaces.com/Flipping+Mirroring+and+Rotating" rel="nofollow noreferrer">here</a>.</p>

<p>UPDATE:
As Jossie has since pointed out, his solution is correct for the question asked.  My solution is for when you need to transform a board (oriented with white at the bottom and black at the top) in which it's black's turn, to one in the same orientation, but seen as if it was white's turn.  An example of when you'd use my solution is converting a database of boards to one where they're all being represented with the same orientation, and same player to move.</p>
 13147: <p>Although one may try to place the first row as the eighth, the second row as the seventh, etc., the board will appear flipped (as shown above).</p>

<p>This is a horizontal flip (mirror-image over the x-axis) and does not look quite right. Switching the letters from lower-case to capital and vice-versa would switch the colors, but that's not the best solution. </p>

<p>Upon closer examination, a vertical flip (or mirror over the y-axis) of this horizontal flip would solve the problem: Reverse each sub-string delimited by a slash until you hit the second field, then leave it as is.</p>

<p><strong>The final product is the same FEN, except the first field is a reverse string of itself.</strong></p>

<pre><code># r4rk1/p3ppbp/2n3p1/8/3q2n1/N1N5/PPQ2PPP/R1B1K2R w KQkq - 0 1
</code></pre>

<p>Board:</p>

<pre><code>r4rk1/p3ppbp/2n3p1/8/3q2n1/N1N5/PPQ2PPP/R1B1K2R w KQkq - 0 1
</code></pre>
