Sorry for your loss.
You can either disband the current Family Group, and create a new one yourself so you can add payment methods as you require, or you can request access to your husband's account and keep it as the organizer account and update the payment method there. Might not be possible, if underage children are involved. Otherwise, everyone can just elect to leave the current Family Group and join the new one you create.
The second option may be less disruptive and will also let you manage the account which you would need to do anyway as all active purchases and subscriptions made by the organizer would remain tied to it. You would need to subscribe to everything again on the new Family Group and go and cancel everything on your husband's account any way.
To request access to his account see the following support document.
click here ➜ How to request access to a deceased family member’s Apple Account - Apple Support
Then you can manage the Family Group from a device signed into his Apple Account.