Making non main branch main branch to Fork/export

Hi, I want to make my branch named “CU_Backup” main branch.

Solution Tried:
I tried to merge my 2nd branch into main, but still main do not contains all things from 2nd branch.

So is there any way I can make my 2nd branch as main or something.

Main Goal:
I want to fork/export the project as well. And while doing fork/export only main branch is used…