Update the needed file only.

Hello!
I’d like to update my theme but only the file needed on the update 1.6.1 of the theme.
“- Fixed BBpress error in header
/header.php”

But i don’t want to reinstall the theme because i modified the theme a lot, and if i reinstall it with the update i’ll lose all my modifications.

Is there a way to update the theme without reinstalling the theme completely ?
Thanks in advance for the help