In Bitbucket Cloud, please go to [Your Repository] >> Settings >> General >> Repository details >> Update repository details >> Main branch.
At the same time, I would suggest you on having a look at Branching a Repository, focusing on The Branching model which describes the DEVELOPMENT label and PRODUCTION (disabled by default).
More information is also available on Weird square appeard on Branch site.
Hope the above answer to your inquire.
In my experience, I was able to change this setting from the General > Repository Details sidebar menu tab. In the main content area, there is an Advanced section that is closed by default. Open it and there are other settings, including one for "Main branch"
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