You can only select a personal, current account as your Main to Plum (e.g. you can’t use a credit card as your main account).
To switch main accounts, go to the Linked Accounts list on the Homepage, select the bank account you’d like to use and then tap ‘Make Main Account’.
When switching your main account, we’ll need to cancel your existing Direct Debit mandate and create a new one for the new Main Account.
This means it may take an additional 2–3 days for your next saving to be made, before falling back into the usual pattern.