Account Recovery
Make sure you are completely logged out of Steam before you start the recovery process

https://steamhost.cn/help_steampowered_com/en/wizard/HelpWithAccountStolen
https://support.steampowered.com/kb_article.php?ref=2347-qdfn-4366

When doing an account recovery, do not try to rush thru the process. Trying to rush thru normally results in information being missed and time wasted. Take your time and read everything.

Here is a guide that another Steam user was kind enough to make. It will help you navigate your way thru the recovery process if you are having any problems.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1126288560

keep in mind that steam does not recognize the transfer of ownership of an account. if the account was your fathers and he has passed, then you do not have the right to access it.

companies may have policies about the passing of an account owner and what conditions will allow them to transfer the ownership, but that is normally kept internal to the company to keep people from trying to game the system.

may be best to just try to recover the account and not mention that it belongs to your father or that the owner has passed on.