Accessing a deceased parent's iphone X without losing data.

So, my father passed away a month ago and my sister sent me his iPhone X. She tried to gain access to it by guessing his pass code, but the phone's security eventually locked her out. So, I am now trying to ascertain the proper procedure - via Apple - to have his iCloud account transferred to my name, so that I may access his phone's data without losing it. He used his phone to store all of his passwords and account info required for fast-tracking discovery for probate, so if we lose everything, including precious photos and videos, then what's the point, really. Can someone please tell me what the procedure is and how long I should anticipate waiting? Thank you in advance!