There's three ways to interpret your question:
- You only have the file in a subversion repository, and you want to modify its contents
- You have the file both in a repository and on disk, no changes done yet, and you want to modify its contents both places
- You have the file both in a repository and on disk, the file on disk contains changes, and you want to update the repository version with those changes
If the first, you need to first check out the file to a working copy on disk, then you move on to case 2.
If the second, you need to modify the file on disk with the changes, and then move on to case 3.
If the third, you need to commit the changes to the repository.
You do this with your chosen Subversion client. If this is the command line, a typical command looks like this:
svn ci -m "Text that describes the changes that were done"
The real problem here though is that you don't know how to use Subversion. I would find a Subversion tutorial and play around with it, create a temporary repository and put files into it, play around with the commit, checkout, revert, log, etc. commands to get a feel for how the system works.