Using SAS' proc sql or data step to replace the value of a single cell in a table?

Let's do temporary copy of SASHELP.CLASS data for performing an exercise.

data class;
  set sashelp.class;
run;

Select one string value

%let new_value=90;
proc sql noprint;
   select weight into :old_value from class where name='Thomas' and Age=11;
quit;
%put --- f(Tomas,11) = &old_value. ---;

Update in SQL is pretty simple, you have got conditions:

proc sql;
  update class set weight=&new_value where name='Thomas' and Age=11;
quit;

PROC SQL could be used for update more than one table via VIEW, but it's another question.

In the data step you can use transactions (but data sorting is required, which is not free). Let work with initial copy of CLASS dataset:

proc sort data=class;
  by name sex age;
run;

Prepare example transaction data (could be more then one records):

data transaction;
  set class;
  where name='Thomas';
  weight=&new_value;
run;

proc sort data=transaction;
  by name sex age;
run;

And make update:

data class;
  update class transaction;
  by name sex age;
run;

Keys name, sex and age here are just for example.