Change research supervisor

My actual supervisor is the senior mentor (haven’t communicated with them), and I’ve been assigned a junior mentor A, but I want to do research with another junior mentor B in the group who has better credentials. What should I do? Senior students, any successful experiences? By the way, is it still possible to change my real supervisor (if switching to junior mentor B doesn’t work)?

It should be:
The nominal mentor is the big boss
The actual mentor is the small boss A?

If that’s the case, if you want to go under B, you first need the big boss’s consent, right?

That’s right, I just don’t know which reason would be best.

It is recommended to directly talk to your senior advisor, but first prepare background information on each advisor’s research areas and express your determination and interest in working with the intended advisor; senior advisors generally will agree.