Well you could use the informat YYMMn. This will set the date value to the first of the month in question.
So in your case May 1, 2013
Here is an example that presumes that your variable with the '20135' value is named mydate.
proc sql;
create table test as
select *, input(mydate, yymmn6.) as mynewdate
from mydataset
;
quit;
I hope that this helps you.
Klaz