How do I transfer deceased spouse’s iMac to my Apple Account?

My wife had an iMac, I can access it fine, I have the password to it. I will eventually want to change it all to my needs ie email and just in my name generally. What problems am I going to come across or will it be a fairly easy transition. I don't have her Apple password but I do have access to her phone. Will this present a problem or can I just add my apple account to it. Just any tips would be very useful, I just don't want to go at it like a bull at a gate.

[Re-Titled by Moderator]