SAS Data Step If then else

My predecessor set up this update and I have identified a correction that needs to be made. Problem is I really don't know what I am doing in SAS. The issue is that the below code deletes all records with non-zero reversal id's. It needs to include the records with nonzero reversal id's only if the original record (reversal=0) is in the current month. And in that case it should include the highest reversal id related to that record, and delete all the others.

   if reversal<>0 then do;
     if reversal = 0 then deleteit='Y';
   else deleteit='N';