To change which membership plan is the default:
Click on 'Admin"
Select 'Online Signup Form'
Click on the 'Signup Pages' tab
At the top on 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 is defining 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