Assume the current main branch is named master and you you created a new branch with the name main that should be the new main branch.

Also assume, that you have forked the main repo and you want to switch to this new main branch.

Fetch the new branch information from upstream

git fetch upstream

Checkout the new main branch and remove the old one

git checkout -b main upstream/main && git push -u origin main && git branch -d master

Change the main branch in the GIT repo online. Remove the old master branch in the repo

git push origin :master

Synchronize the data

git remote prune origin upstream