To change a default setting, head to Money > Bank accounts and click the pencil icon, next to the new account to edit it:
On the next page, select 'Settings' from the side-menu:
Then select 'Yes' next to the 'Make default choice in dropdowns' option and click the blue 'Save' button to update:
It should then appear as the default account, when you next go to pay any bills or invoices.
For the old account, if it is no longer being used there is an option to archive it if you wish. If you head to Money > Bank accounts again, click the edit icon, go to 'Settings' and you will see an option to 'Archive' next to the 'Save' button (as seen in the screenshot above). Selecting that will archive the account.
I hope that this has been helpful. If you have any further questions, please do let me know.
All the best,