Why does the wordpress theme that I edited always rollback to its original version?

This is so weird to me. I'm using wordpress 4.1.1 and GeneratePress theme.

I want to change something like header font size and footer content. So, I edited files:

/wp-content/themes/generatepress/style.css
/wp-content/themes/generatepress/header.php
/wp-content/themes/generatepress/functions.php

After I edited, it displays as what I want. However, a few days later (I'm not exactly sure, but maybe 4-7 days) the theme has gone back to its original version. What I edited were gone. I really don't know what made this to happen.

Ps. I tried to do this 2 times. For the first time, I edited the files in my computer and then used ftp. A few days later, everything was gone. So, I tried to do it again. The second time I used admin theme editor. A few days later, everything was gone back to the original version again.