Welcome to the forum. It's hard to know where to start but I'll give it a shot.
1. Open a drawing with a current template, make the desired changes at Tools > Options > Document Properties.
2. Make sure it's using the desired sheet format(s), which may involve editing a current sheet format and saving it if you don't already have one. Sheet formats are a different file type from drawing templates.
3. Save the drawing template at the location specified at Tools > Options > System Options > File Locations > Document Templates.
There is more information in this discussion, along with many others here: https://forum.solidworks.com/thread/57204.
I know this is very general information, but your question is also very general. If you have more specific questions I, or someone else, will be glad to help.