Google Search Console

Hi All
Firstly let me say I’m very much a novice, so be gentle with me lol.
I’ve done a site for a friend replacing her old one and it is in the
Google search console of the original web designer.
The sitemap is now all wrong and I cant verify I am the new owner.
How do I go about this?
Sorry for being stupid, but I have no idea what to do?
Thanks
Jim