How to upload a template in Craft CMS 3 on an Ubuntu web server Really new to this platform and could use some help with how to upload a theme/template which I downloaded from ThemeForest.  The files are compatible with HTML 5 and Craft 3 is on an Ubuntu server.
The answer will depend on the extent of your web development skills.

Do you have access to the web server via FTP?
If so, upload the template files to the templates folder where you installed craft. The templates will have to be edited to bring in your sections, entries, and content that you add in the Craft dashboard.

Check out these docs to learning templating and converting templates to twig/Craft templates.

https://docs.craftcms.com/v3/dev/