So it's safe to assume that to force clear browser cache via FTP (without SSH) after a flarum update or new plugin installation do the following.
- go to the assets folder and delete the jss, css, and rev-manifest.json files only.
- also clear the storage/cache directory
NB: Make updates to flarum or add new plugins during hours when you know traffic to your forum is less, so that your users won't notice the initial error message (in red background), before you perform the above instructions. ?
Thanks again jordanjay29 @luceos thunder