To change which membership plan is the default:
Navigate to the left-hand sidebar and click on 'Admin'
Select 'Online Signup' from the Admin menu
Click on the 'Signup Pages' tab
At the top of this page, it will say 'Available at https://yourpractice.hint.com/signup which will redirect to..' and there will be a drop-down menu. This is what defines your default membership plan and which plan your generic signup link goes to
Select the drop-down menu and pick which plan you would like to set as the default