Step 1

git fetch
git rebase origin/master master
git branch --move master main
git push --set-upstream origin main

Step 2: Set up main as the default branch in GitHub

• go to Settings tab
• click on Branches left menu option
• select ‘main’ as Default Branch
• click 'Update’
  Step 3: Reconfigure Heroku to deploy from main
  ===============================
• go to Heroku automatic deploys
• select ‘main’ in the Choose branch to deploy combo
• click on 'Enable automatic deploys’
  Step 4: Delete the master branch
  ============================

git push origin --delete master