How do I permanently change tab stops in an existing text document?

FORMATTING ISSUES
I began writing a film script, now at one-hundred plus pages and need to clean up the formatting:
The HELP program dosn’t show how to permanently change TAB STOPS (every time I moving them, delete or create and save the document they just reappear in the same old spot.
The Margins continue to display in metric rather than inches, I need to permanently change them to the latter, as well.
Can you help explain where the information lies? Or, how to actually make these changes?

Also, it would be nice for US users if tab stops could be switched from metric default (cm) to imperial (half-inches).

Please take a look to documentation:

Working with styles

Controlling tab stops - Page 15

Please note: I followed the directions for setting tabs on Page 15 of the “Working With Styles” to the tee, including, erasing the old tab stops. I also changed my desired measurement from Inches to Cenimetres (because that seems to be the program’s desired default. I saved the document, closed it, then reopenned it: all of the original locations were back in place. So, the answer does not seem to be under “Working With Styles”.
Could the LibreOffice program I downloaded have a glitch?

This is happening to me. Change tabs save close and reopen and we are back to square one. I guess this is a bug in the prog.
This issue wasn’t answered successfully it seems!

Have you tried to modify the “Default Style”? In the document, bring up the “Styles and Formatting” menu (F11). Right click “Default Styles” and choose Modify. From there, you should be able to change the Tab settings for the document.

As far as the Metric/English measurements go, I don’t have an answer for you.

I tried that for a second time and again, once I save the document, or closed it out and then reopenned it, all the formatting had changed back to the same default settings. This program does not want the default changed, for any reason.

A trite advice:

Might it be that your user profile is corrupted? Did you already try a new one following this guide?

Nope, that didn’t work.