Yes, you can pay off your installment balance at any time. You can even make one or multiple installment payments in advance. Just make sure there are no pending orders/requests being processed and your account is current with no outstanding payments.
Log in to My Account and under the Account menu, select Devices & Plans. On the Devices page, you'll see the progression of your installment plan and how many payments you have left on the device. Click on "Manage Installment Plan". You'll see options to either make a partial payment or pay off the device in full.