Please help this rubbish tech parent! Microsoft log in for Minecraft not recognised by Mojang

Hello!

My son loves Minecraft and the account he uses is logged in via the Microsoft log in page, which takes you straight into the game. He plays on an Apple ipad. He wants to change his Minecraft username and to do that apparently (according to web tutorials)
i have to go via the mojang site. However the Mojang site won’t recognise the microsoft log ins for minecraft and i cannot seem to get anywhere. I have spent so long trying to navigate this and i’m losing the will to live! My son is desperate to change the
name and i want to help him out, if anyone can tell me what i need to do i will be soooooooo grateful!

:)