Go to https://app.walls.io/me#profile and enter the email address of the new wall owner. After you are done setting up the wall for the next person, log out.
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The new admin then has to go to the login window and after entering their email address, they should click on "I forgot my password" to receive an email to set up a new password. 

Please be aware that changing the account owner does not change the person who pays!

Go to https://app.walls.io/me#subscription and click on "Edit my address/edit payment information":
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In the new window, click "Subscriptions" > "Manage" > "Update payment method":
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