data final;
  set a b;
run;

proc sort data=final nodupkey;
  by id year ym;
run;

assume you want to keep the original B, just use the variable in A file to replace

proc sql;
  create table new as select distinct b.id, b.year, b.ym, a.*
  from B as a left join A as b
  on a.id=b.id & a.year=b.year, a.ym=b.ym;
quit;