Account Ownership and payment method change due to husband passing away

I’m very sorry for your loss. There are two separate issues here: changing the payment method and changing ownership of the Microsoft 365 subscription.

If you still have authorized access to the Microsoft account currently holding the subscription, you can change the payment method here:
account.microsoft.com/services

However, Microsoft does not allow an existing Microsoft 365 subscription to be transferred from one Microsoft account to another. If the subscription needs to be under your own Microsoft account going forward, turn off recurring billing on the existing subscription, allow the current subscription period to expire, and then purchase a new Microsoft 365 subscription using your own account. For Microsoft 365 Family, the new subscription owner would then need to share the subscription with the family members again.

If you do not have access to your late husband’s Microsoft account, Microsoft has a specific process for accounts belonging to someone who has died. Because of privacy and legal requirements, Microsoft Support cannot simply change the account owner or provide access to the account.

https://support.microsoft.com/accounts-billing/manage/accessing-outlook-com-onedrive-and-other-microsoft-services-when-someone-has-died