Substitute Characters in a Defined String

[kfbaker0206]

I am trying to change the 4th character in a string. The 4th character consists of numbers and letters. The code that I used changes the entire string to all x's. Please see code. Any assistance will be greatly appreciated. 

Data Want;
	set DATASET1;
do _N_=2 TO length(code);
substr(code,_n_,4)=byte(mod(rank(char(code,_n_))+&offset,255));
end;
run;

This is the original data

DATASET 1

CODE

0S124

0R123

0T435

0F5F3

0R4RY

This is what I want

DATASET 2

CODE

0S1X4

0R1X3

0T4X5

0F5X3

0R4XY

Here are my results

CODE

XXXXX

XXXXX

XXXXX

XXXXX

XXXXX

[Reeza · Accepted solution]

This works. Use the Left of = SUBSTR function. Not sure why you're looping anything or using _N_.

data want;
set sashelp.class;

substr(name, 4, 1) = 'X';

run;

[kfbaker0206]

Thank you! I had to change the 4 to 6. Other than that it worked!