The next stage of the upgrade is to upload the files. There are two options open to you:
It's easier to overwrite the old files, so that's what we'll do here.
Before you do this though, one thing—close the forum! This prevents users from logging in and allows you to work in peace. It also means that you and only you see the errors if things go wrong.
Log into the AdminCP, expand vBulletin Options in the left-hand menu, and click on vBulletin Options.
In the right-hand pane, double-click on Turn Your vBulletin On and Off. Change Forum Active to No, and click on Save. Now only Administrators ...