The way I remember it is as an English sentence:

rebase (from) old_parent (up to) branch_tip onto new_parent

That is:

git rebase old_parent branch_tip --onto new_parent

(Note that the arguments are in a different order to how some people write them; the --onto new_parent is an option, not a positional argument, so can go anywhere, and I just find it more logical at the end rather than the beginning.)

In your case:

• old_parent is the commit hash shown on your diagram as H
• branch_tip is branch-B (and defaults to the currently checked out branch)
• new_parent is I AKA origin/main

So look up the actual commit hash for commit “H”, and run:

git rebase H branch-B --onto origin/main

Or:

git switch branch-B
git rebase H --onto origin/main

Or if you prefer to put the --onto first:

git switch branch-B
git rebase --onto origin/main H