The first thing to do is to backup your entire forum. All of it. The database and the files.
Once you have done that, you can run the auto update
routine, and it works for about 95% of the code (depending on your existing mods).
Just double check everything before you commit any changes. I found it best to download the new merged files, then upload them. It took me about 20 minutes from start to finish, including double checking file differences. However you will have to manually edit any styles you have that aren't proSilver or subSilver 2. There are no phpBB SEO style code changes from 3.0.9 to 3.0.10, but there are phpBB code changes
that need to be done.
This link may be helpful: DiffNow - Compare files online
Failing that, you could reinstall the premod from scratch and reinstall your mods. Then import all your posts, groups, users, etc.
If you have backed up all your files and the database in the first step, if anything goes haywire you will be able to easily restore your forum.