In many cases the templates downloaded from the site will be opened as read-only for security reasons. When you open the file, you should see a banner across the top like this:

You can then click on the ‘Enable Editing’ button.

If you don’t see the banner allowing you to enable editing, open a file explorer and right click on the file name, then select ‘Properties’. At the bottom of the general tab you should see:

If you see the Security section, click on Unblock. If you don’t see the security section, make sure Read-only is not checked. If neither of those work, there is a different problem that I don’t know how to fix.