I need to replace the 45th position in a string of lenght 67 and keep getting syntax errors.

are you doing it in 4gl, store proc, or sed command?
please post your code

I'm trying to do something like this:

set priority = replace(substring(priority from 45 for 1), 'n','8');

I'm not sure if I'm on the right track. Basically I want to replace whatever is in the 45 position with the value 8. The entire string is 67 long.

I'm using SQL Editor to update an informix db.

Thank you

try using
set priority[45] = 8