I’m sorry for your loss. To guide you correctly, could you please clarify whether you can currently sign in to your own Windows account and whether that account is an Administrator or Standard user? Also, when you mention changing the PIN, are you trying to change the PIN for your own Windows account or for your late partner’s account?
Please also let me know whether the Microsoft 365 subscription is Personal, Family, or a business/work subscription, and whether you already have your own Microsoft account. If possible, check Settings > Accounts > Your info and Settings > Accounts > Other users and tell us what types of accounts are listed, without sharing any email addresses, passwords, PINs, or other personal information.