i was wondering if i pay my back child support in a lump sum can i get it reduced

The court will not reduce your child support or arrears simply because you paid your back support. It is already established that you owe the back support, so paying it does not change anything except that you now are current.  You can perhaps reach an agreement with the other party to whom you are paying support, that if you pay the back support, they agree to a modification of support, bur only if the other party agrees.  If the other party will agree you can sign a joint stipulation and submit it to the cout.  Normally ongoing child support must be in accordance with the Florida guideline worksheet, and courts must articulate a reason for deviating from the guidelines. Further the court has the ability to direct the clerk to adjust the arrears based upon a stipulation between you and the receiving party. For example if the other party agrees to accept 50% of the arrears owed, and also agrees to the entry of an order zeroing out the remaining arrears.