If your 'guilty plea' was within the last ten days, you should try to withdraw the guilty plea by telling the District Justice office you want to plead not guilty. Some offices will let you; some will not.
If that does not work, or if it is outside of ten days, you have 30 days from the date of conviction to file an appeal at the Clerk of Courts Office in the County Courthouse. That wlll get you a new hearing date at the Courthouse. There should be 'diversionary programs' that allow you to avoid a conviction and the license suspension.
If neither of those work, remember that once your supervision is over and your fines and costs are paid - you can have that underage consumption removed from your criminal history by filing for an expungement.
The paying of the fine amounts to a guilty pleas. You have 30 days to file an appeal in the Court of Common Pleas. If 30 days have already gone by, you can file a late appeal and ask the Ct. to here the appeal. It is important that you hire an attorney to protect your rights.
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