How can I uninstall then reinstall Mail?

Hello,

I used to see an Inbox, Junk folder, sent folder, deleted folder and draft folder under 'On My Mac' in Mail but now only see the sent, and junk folders (see attached image). The emails used to come into the missing On My Mac inbox and now go to my server's email box within Mail which I wasn't using (see second attached image - don't worry about the number of emails I have!).

All my sub folders are under On My Mac so now the searching tool can't pick up everything and it's harder to access and clear out my inbox, and the moving tool is limited to a few folders. How can i get the old format back? Do I have to restore an earlier version via Time Machine or uninstall, then reinstall Mail? If so, please advise what steps I need to follow. Many thanks for your help.