You’ll be able to upgrade your subscription plan from Printful Plus to Pro at any given time, but to start enjoying your Printful Pro plan, you’ll first need to wait until the current billing period for Plus ends.
To upgrade, go to your Printful Dashboard > Settings > My account > Subscription plan. Click on the Upgrade button next to the description of your Printful Plus plan. That’ll take you to another window and, once you’re there, click the button Switch to Printful Pro.
If you wish to change your monthly plan to a yearly one (or vice versa), then cancel your existing subscription and resubscribe to your chosen plan.