Rebase a branch onto another branch from a specific commit

[ekans]

When I work on 2 branches (b1, b2) and b2 is based on b1, I use to rebase b2 on b1 when b1 is updated.
In lazygit it's possible to rebase b2 on b1 but if a b1's commit has been modified I like to rebase b2 on b1 like this:
(b2 adds 2 new commits after b1's commits)

$ git checkout b2
$ git rebase --onto b1 HEAD~2

To be clear, I would like to say: rebase this branch from this commit onto this branch.
Is it doable with lazygit?

[ekans]

I answer to myself.
Using git rebase with --update-refs is the solution I was looking for.

[ekans]

(just works in lazygit by enabling it by default)

[akaihola]

So what are the actual steps to do this?

If I understand correctly, this is the same as "Rebase subset onto" in Magit.

[stefanhaller]

Navigate to the commit below the first one that you want to include in the rebase; hit shift-B; select the branch that you want to rebase onto, hit r.