- Clone a template set
- Set the cloned templates to the 'default' set in Admin > Preferences > General
- Set 'Update module template .html files from themes/your theme/templates directory?' to 'yes'
- Go to System Admin > Templates > Module > whatever.html > Edit
- Make changes, hit submit
...& thats it, no updating modules or erasing templates_c.
Templates used to baffle me, I would download the entire 'templates_c' folder & edit the html files & then overwrite the edited template files directly - which was a tremendous pain vs the method described above. Still have issues exporting/importing templates, sometimes I will export & think I have a backup & go to use it & find that the exported gzip has 0 bytes... but sometimes it works.
*shrugs*