Your default membership plan is defined by your default signup link.
To change which membership plan is the default:
Navigate to 'Admin' > 'Online Signup'.
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 which plan your generic signup link goes to, which defines your default membership plan.
From the drop-down menu, pick which signup URL you would like to set as the default. The plan associated with this URL will be your default signup plan.