HOW TO UNDO WORDPRESS CHANGES?

I am not talking about post/page revisions/restore...



LET'S SAY I HAVE ADDED 1 PLUGIN AND IT BREAKS MY SITE SO FURTHER I WILL DELETE
THAT PLUGIN BUT THE IMPACT OF THAT PLUGIN STILL REMAIN ON THE SITE...



So, now... I want to restore the previous version before the plugin added...so
how can I do it?