I know main branch can be renamed in SonarQube CE via web API but it will be useful this can be done from a option added in a project view.
Do you plan to add this feature in future releases?
Kind regards,
Welcome 
this feature ain’t available yet but it’s tracked at the Sonarqube productboard under
https://portal.productboard.com/sonarsource/3-sonarqube/c/169-allow-a-new-branch-to-become-the-main-branch
So Dude grab your white russian and vote 
Though i don’t know if this affects the CE also as the branching feature
is part of Developer edition and above.
EDIT
After the update to Sonarqube (Enterprise) 9.5.0 yesterday I just see it is now possible to rename the main branch via Sonarqube web ui
Gilbert