Just to note first, you cannot automatically upgrade or downgrade from a non-Plesk VPS to a Plesk VPS and vice versa. If you want to do this please contact our Support or Accounts department.
If you are on a VM then you can remove or add plesk by choosing the appropriate template. See here for changing templates:
You can upgrade or downgrade the VPS package or 'plan' in your main control panel at:
Billing >> Subscription Management >> *subscription name*
Here you should see a "Switch Plan" button. In here you can select what plan you want to upgrade or downgrade to.
Note: you can not downgrade VM plans yourself. Please contact our Sales department if you wish to do this.