I bought this template but I don't know how to edit its files on my computer. It seems the file extensions are .tpl
How can I edit them like HTML files and then upload them to the server?
Whmcs (a webhost and billing manager software) uses the .tpl files to allow for customisations. There is no way to edit them like html, that I know of.
Tpl files basically need a bit of trial and error in modifying them.
The best way would be to install a trial version of Whmcs or if you have it already installed, perhaps install a second development version (you can apply for a free development license from whmcs).
Then use a txt editor like notepad++ (or another html editor software) to edit the tpl files - then upload to your test site - tweak and repeat as needed... yes, it's not user friendly - but that's whmcs for you.
The whmcs documentation gives a helping hand too.