How to Assign Blog to New Domain?

I just created a new domain (same host) and got the redirect done, so I can enter the old domain url (https://healthempower.info) and land on the new (https://bodymindpeace.com) domain.

BUT the host didn’t tell me I would also have to change something within WP. Now I can’t log in to my site admin – I just get directed to the home pg.

Please tell me which file(s) I need to edit to reflect the new domain, and how I can get in there to do it. (someone else set the site up – and he’s not available now)

Many Thanks!
Dane