http://mponpsecu5qaqu4s7chzbldsp6hu45pwecvr72m26xbf6lo3slqo6tad.onion/docs/INSTALL.html
Next, delete all the existing phpBB files, you should leave your files/ , images/ and ext/ directories in place, otherwise you will lose your file attachments, uploaded images and get errors due to missing extension files. You can leave alternative styles in place too.