My extension was accepted but I realize I made a mistake on it by putting 0 on tax liability. I will be getting a refund so I thought tax liability would be 0. Can I correct this? Everything is 0 excepted for the amount that has already been paid in.
There is no amend to an extension. An extension only extends the time you have to file your return, so as long as you put the correct information on your return, you will be OK. Any taxes owed must be paid by April 15, 2019 to avoid a penalty & interest.
If you do not owe any taxes at the time of the extension, Form 4868 line 6 should be 0, but you are referring to the tax liability reported, line 5. That is not as important as line 6, ehich you entered correctly.
If you were to owe tax in the future,there are several ways the IRS accepts payment including installments: https://ttlc.intuit.com/questions/1901676-how-can-i-pay-my-federal-taxes
Here is a resource to see how much the penalties and interest could be if you did owe...it compounds daily. This link looks like it is not for individuals, but it is for individuals. https://www.irs.gov/businesses/small-businesses-self-employed/understanding-penalties-and-interest