You can use either the IRS2Go app or the Internal Revenue Service website to make payments towards your taxes, even if you do not have an IRS.gov account, and even if you've already filed an extension but haven't yet filed a return.
General instructions: "Make a Payment", "Pay Taxes", and "Extension" are the key words and phrases you're looking for.
The specific navigational instructions below for the website are good for 2017. In future years the specifics may change, but the general workflow should be similar. I have not used the IRS2Go app, so I can't describe its workflow, but it is supposed to have this feature as well.
Choose the "Make a Payment" option from the main entry page.
Find the "Pay Your Taxes Now" box in the middle of the page and choose your method of payment. "Direct Pay" debits from a checking or savings account and does not charge a fee. "Debit or Credit" allows you to use plastic, but charges a fee.
Choosing "Direct Pay", you then select "Make a Payment". On the next page, under "Reason", select "Extension", which will populate the "Apply Payment To" dropdown with the 4868, and the "Tax Period for Payment" dropdown with the most recent year. You probably don't need to change either of these.
Clicking Continue takes you to a series of pages where you can enter and confirm your identity and account information. You'll have the opportunity to specify the amount to pay, so it helps if you remember or have at hand the amount that you still owe.
When you're done, a confirmation page will be displayed, and you'll be sent a confirmation email.