Queries :: Replace 2 Characters In String With 0s

I'm familiar with writing a replace query to replace characters or strings but what I'm trying to do this time is a bit unique to me. I have a string of numbers that will either be 8 or 9 digits in length. The first 1 or 2 digits will be the State code (1-50 hence the discrepancy in number of total digits), then a 2 digit agency code, then the last 5 digits are a producer code. What I need to do is change the 2 digits for the agency part both to 0. So basically characters 6 and 7 if you're counting from the right. To me it feels like I'm trying to do it backwards so I'm having a hard time writing it.