Magento – How to do a SYSTEM rollback manually

this morning I did a system backup (backend -> system -> tools -> backup) of my store because I wanted to try some extensions and now my site is ruined.
I've stored the backup on local, so how can I make the rollback? I have to overwrite all my magento installation with the backup ?

I can't access to the admin panel, so if any one knows for sure how to make the rollback manually with the system backup I'll appreciate it so much.

Thanks.

EDIT: I solved the problem that had ruined my store. It was appearing error 503 and lot of errors from the processor.php file. I've deleted the maintenance.flag file from root dir, and now works.
Anyway If you want to answer the question in order to help other people that needs to do that, you're free to do.