Can a property settlement agreement be amended if both parties agree

Nearly all agreements contain a standard clause that the agreement may only be modified by another written agreement of the parties. Even without such language, all agreements can certainly be modified by written agreement, as that is the very nature of agreements. However, the court should have incorporated your agreement thereby making it also a court order.  To achieve the same effect with the modification, I strongly suggest the agreement be framed as a stipulation with court approval.   If you live in the Delaware Valley, I am more than happy to advise you on this, or you should seek the advice of local counsel.