How to change the directory from /mybb to /forum with no errors?
First I installed the mybb at a folder named mybb,and now I want to rename that folder to forum
Change your settings first to fit the new change.
Then create a new directory "forum" and move all your files into that forum, simplez.
First, you need to go to ACP > Configuration > General Configuration > and update your Board URL and cookie path, i.e. change the mybb to forum. Then, you can rename the mybb folder to forum and it should be OK. You may need to clear your browser cookies after you rename the folder, though.