You are asking in this thread and others about reinstalling Joplin in case it can fix the problems you seem to be having.
If you are not happy with your local install and believe that the sync files on Dropbox are fine there are two main ways to "refresh" your local client. The third option below is just the second one with an added uninstall / reinstall stage.
1 - Use the Joplin settings
The "Delete local data and re-download from sync target" option will do just that. Any plugins and customisation will remain intact and the client will still be connected to your sync server (Dropbox). It will delete your local note and attachment data and download it again from Dropbox. If you have a lot of notes and many large attachments this will take some time.
2 - Reset ALL local data
However if you really want to "reset" your local client you can do the below and it will reset everything.
- Fully QUIT Joplin. File > Quit
- Rename the folder C:\Users\Username\.config\joplin-desktop to something like C:\Users\Username\.config\joplin-desktop-old
- Start Joplin
- Joplin will create a new joplin-desktop folder and a new local database within that folder. It will be as if Joplin was just installed, with no plugins, no customisation and only the default "Welcome to Joplin" notes. Joplin will not be connected to the sync server (Dropbox).
- Delete the welcome notes if you want to.
- Go to the sync settings in Joplin and connect the client to Dropbox just as you did before.
- Let the client pull down all the notes. If you have a lot of notes and many large attachments this will take some time.
- Do not delete C:\Users\Username\.config\joplin-desktop-old until you are happy all is working correctly.
3 - As option 2 but with an uninstall / reinstall
Between steps 2 and 3 above, uninstall and reinstall Joplin
You now have a fresh Joplin install with a fresh copy of your note data from your sync server. Whether this fixes your problem with tags is another matter.