You can't. The only option you have once you've engaged in EDGE is to pay a downpayment (reducing the balance) at time of purchase or to pay it off. VZW doesn't allow for you to make additional payments to reduce the balance.
Can I pay more than my scheduled monthly Verizon Edge installment?
No, unless you are either seeking to Edge Up to a new device, which requires that at the time of the Edge Up transaction you pay at least 75% of the device (50% for customers with an Edge Agreement entered prior to 6/1/14, 60% for customers with an Edge Agreement entered between 6/1/14 and 10/15/14) following the initial 30 days of the Verizon Edge Agreement; or you are seeking to pay off 100% of your Verizon Edge Agreement, which you may do at any time.
Since it is a no interest loan, just pay yourself the extra and hide it in an account or safe place until you have enough to pay in full.