You can update your credit card from https://yourcommunity.mobilize.io/main/settings/billing (or https://community.yourbrand.com/main/settings/billing if you're using a Custom Domain). Navigate there via your Community Settings. You'll need to be the Account Owner in order to access billing. Community Managers do not see the Billing tab at all.
In your Community Settings, head over to Billing, but you don't need to update your plan. There's a light gray link to simply update your credit card. It says Change payment method. Here's what it looks like in our test account below.
After you change the credit card, our system will automatically retry your card within 3-7 business days based on an automatic schedule. It won't retry your card immediately. If you need the charge to go through sooner, please send a quick chat via the platform or email us at firstname.lastname@example.org.