OK.
I found a quick-and-dirty temporary solution.
As an online education forum suggested, a interesting school science experiment might be to see if adding weights and changing their position on a tuning fork will change the frequency of the fork.
So I played science student, and tried the experiment.
Two big rubber bands on the tines clearly lowered the frequency... by way too much.
Next I tried a couple strips of masking tape wrapped around the very end of the each tuning fork tine.
That also lowered the frequency, by only a bit too much, to 20 cents flat.
So I removed those pieces of tape and carefully measured them.
Then I cut some tape strips about 9/29ths as long (actually a few mm longer).
Wrapped the new lengths of tape on the fork tines, and the fork rang 3 cents flat.
A little more fine trimming of the length of the tape with some sharp scissors, and I got the fork to measure within +-1 cent of 440 Hz, as measured against several calibrated iPhone tuners (both dial and strobe).
The fork also now sounds good when played simultaneously with an mp3 of a 440 Hz tone to check for beats.
The tone probably isn't as pure, and tape will eventually age and wear, but this quick fix should work well enough until my next trip to a good music store for a better quality tuning fork.
music.stackexchange.com/users/1489/hotpaw2
You could tune it up if you have precision tools that can grind off just the right amount of metal from each tine (this would increase the frequency; to decrease the frequency you would need to add metal!) - but practically speaking that means no, you can't do it.
Does it look damaged?
If not it is probably supposed to be a 442 tuning fork.
If you need a 440 you'll need to just buy one.
Update based on the comment that you had bought a 440 fork: Take it back to the shop, get them to check it.
If their check shows it is a 442 then they can replace it.
If they show it to be a 440 then maybe your tuners are out.
music.stackexchange.com/users/104/Doktor Mayhem
It is defective.
Return it for a refund.
There is no practical way to recalibrate it.
The reason people purchase a tuning fork in the first place is to get something that is exactly calibrated already.
music.stackexchange.com/users/None/
Interesting question!
No-one's mentioned temperature, so far.
I'm off to put one in the freezer and another in the oven!
I'll report back!
Just got back from hospital with frostbite on one hand and burns to the other...
but surprisingly the tuning fork stayed in pitch!
My theory stemmed from the British Yard which lives in London.
It is exactly a yard long at a given temperature.
Longer tuning fork =
lower note, shorter = higher, surely?
music.stackexchange.com/users/2808/Tim
As someone else stated, grinding off material from the end of the tines will increase the pitch.
However, grinding material off at the base bend of the fork will lower the pitch: there the material contributes much more to the fork's stiffness than to its inertia.
But it may well affect the purity of tone.
music.stackexchange.com/users/8773/User8773
I had a cheap allegedly A440 fork that showed sharp on all tuners, and for reference, a better-quality fork showed exactly A. By filing a small amount from the outside of each tine near the join, and checking several times, it is now quite exact.
music.stackexchange.com/users/71005/R. reed
A bit of solder at the tip would lower the frequency.
But to get both tines exactly right
, that's tricky to do.
music.stackexchange.com/users/1941/200_success
I just purchased a new 440 Hz tuning fork.  But several different computer, iPad and iPhone tuners say it's almost half a percent sharp (around 442) of Concert A.  Is there any way to adjust the frequency of a tuning fork?
Tuning a tuning fork?
music.stackexchange.com
<tuning><frequency>
music.stackexchange.com/questions/6336
music.stackexchange.com/users/1489/hotpaw2
It is practically impossible to re-tune a tuning fork. There is probably no practical way to recalibrate a tuning fork. However, if you are able to, grinding some metal off each tine will increase the pitch whilst adding some solder to the tips may decrease the pitch.
Grinding off some metal from each tine will increase the frequency of the fork. Adding some solder to the tip of each tine would lower the frequency of the fork. There is probably no practical way to recalibrate a tuning fork.