First, read the English article below.

Don’t be tempted to read the letter before it is time. Seal it in an envelope or tape it closed. This will also help preserve the letter, especially if you don’t plan to read it for 10-20 years. If your letter is digital, archive it or move it to a folder that you can find when it is time to read the letter. If you’ve chosen to write or print a hardcopy of your letter, you need to keep it somewhere that is both readily accessible and safe from the elements. If the letter itself isn’t easy to spot in your everyday life, you’ll need to write yourself a note that will be; otherwise, you may forget about the letter when it’s finally time to read it. You can store the letter in a memory box or even put it in a time capsule. If you keep a journal, consider writing the letter directly in your journal and marking the page, or writing the letter separately and tucking it in between the pages of your journal. Search out and use a program, website, or application that will let you send emails/texts to your future self. Note, though, that this option works better for short-term letters than long-term ones since you can’t be certain that the website or application you use will still be around 20 years into the future. You might consider using a digital calendar (like Google Calendars), note-taking software (like Evernote), or a letter-writing website (like FutureMe). 

Now, please write a short abstract for it in Vietnamese. Abstract:
Dán thư. Cất lá thư ở một nơi an toàn. Dùng công nghệ để gửi thư.