The simple case:
data want;
set people;
if name='Mark' then age=31;
run;
You can change it in the same dataset a number of ways:
proc sql;
update want
set age=31
where name='Mark';
quit;
data people;
set people;
if name='Mark' then age=31;
run;
data people;
modify people;
if name='Mark' then age=31;
run;
etc.