Step 1: 

When you first go to your Meetn room's URL, click the orange button labelled "Hosts & Speakers, Log In".

Step 2:

Log into any of your Meetn rooms as the owner account. This means logging in with the same email and password you used when signing up for Meetn. Continue to complete the Join Flow and enter the Meeting room. You may join normally with or without video, or use the Settings-Only Mode if you're worried about interrupting an event in progress.

Step 3: 

Open the Menu at the top left and select the option Add/Update Billing Info. If you don't see this option, it's because you're not logged in with the owner account.

Step 4:

Now on this menu you can simply enter your new billing information and click Update. You're all set!