1) your agent/REALTOR can help you in this process and should be guiding you
IF you don't have representation, GET IT, you need it (I have one slot available and might be able to help you)
2) what you really need to do is renegotiate the selling price
Your agent should have done a CMA before you made your offer, in order to align it closely with the market value. Either way, though, you'll have a tough time getting the bank to let you pay more than a property's appraised value and why should you fight to pay more?
1. negotiated a Due Diligence Period with a timely right to address concerns by amendment
2. negotiated an Appraisal Contingency Period with a timely right to seek a Price Reduction, and a right to terminate if the negotiation is unfavorable
If not, I hope that your Earnest Money is not in jeopardy.
If not, another appraisal is not an easy proposition.
If not, depending on the structure of your loan and your lender's underwriting guidelines, you can always pay more from your own funds to allay the difference.
If you can't afford the alternative, I hope that your agen t did their basic job.
Unfortunately, hope is not a strategy.