Thermostat Replacement

Vehicle info: 03 Blazer, 4dr, 4wd, 60k miles

I have some questions about replacing a thermostat on a 03 Blazer:

My SES light came on. I had the code read at auto zone and it indicated a problem with the temperature/thermostat...
It said:
P0128 - Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)


I got the new thermostat and gasket and have instructions on how to replace it.

I dont think the coolant has ever been changed but the levels have always been good.

My Questions:
When I replace the old one should I drain the coolant and replace that as well??

If I do change the coolant is it necessary to completely flush it or can I just drain it from the radiator and install new coolant?

How much coolant will it take to refill the system if I do drain it?

The manual says to use DEXCOOL coolant...Is this readily available at stores like walmart or is it something I need to get from the dealer?

My instructions are for replacing the thermostat are general instructions, does anyone have any blazer specific instructions or tips for me? (I couldnt find the hayes manual for an 03 blazer).

Thanks in advance
jmat

My Questions:
When I replace the old one should I drain the coolant and replace that
as well??


If I do change the coolant is it necessary to completely flush it or can I just drain it from the radiator and install new coolant?


Do a full flush if you never done one before


How much coolant will it take to refill the system if I do drain it?

the manual say the right quantity,


The manual says to use DEXCOOL coolant...Is this readily available at stores like walmart or is it something I need to get from the dealer?

dexcool is available at walmart and other autoparts stores, make sure it is the orange coolant