How do you have pending charges dropped after signing plea?

The question is somewhat unclear. Once you have signed a 'plea agreement' the charges are no longer "pending" because you have already signed a plea agreement. By signing the plea agreement you have, in effect, pled guilty to the offense(s) for which you were charged, or a reduced version of them.

You (or your attorney) can file motion to withdraw your plea if you wish, but by withdrawing the plea you once again make yourself subject to prosecution for the original (un-reduced) offense.

If you are asking about how to have the charge(s) removed from your public record (expunged), you will have to look into the procedure that is in effect in your state.