Merge child branches when parent branch is merged

My main branch is called "main". I then created branch A. After completing what had to be done in branch A, I created branch B from A. Now A has been merged into the main branch and B just doesn't have its parent. How do I update so that branch B has the correct changes from both main branches (which now includes A).

Thanks.

Here's the detailed step-by-step pseudocode plan:

• Switch to branch B.
• Rebase branch B onto the main branch.
• Resolve any conflicts that may arise during the rebase.
• Continue the rebase process after resolving conflicts.