The thermostat is stuck at 260. How do I reset

Sounds like the sending unit.
That is where the wire plugs in for the temp gauge, on or near the thermostat housing.

Normal operating temp for the I6 engine is 210 degrees, so 260 is a little high. Make sure it has coolant....rent a coolant system pressure tool to introduce pressure into the system. You want to rule out any leaks. If no leaks, more than likely your thermostat is stuck closed. It's a PITA job, but while you're in there should replace the temperature sensor located right above the thermostat.

One thing at a time.
Once the engine cooled down, how long did you run it before the engine heated up ? The thermostat could have stuck closed.
Or the water pump impeller may have failed.
Have you checked the computer for trouble codes yet ?

white smoke means coolant is getting into the combustion chamber of the engine which would mean your head gasket is bad. I would change your thermostat too. it doesnt sound like it is working properly. your thermostat is probably getting stuck closed and that is what is leading your temperature to get high. The high heat, or even if your truck over heats a few times, it ruins the integrity of your head gasket.

No there are no reset buttons on any of the guages. Does it read 260 always? if so try taqpping on it lightly and if it still is stuck, replace thye guage.

260 is too high, find that leak, if it is indeed leaking. Temperature should read 210 for normal operation. If your coolant level is normal, replace your thermostat as it is probably stuck closed. Could also, be a faulty water pump, so if it is leaking, this may be your problem too. Anyway, some things to check.

operating temp should be from 195 to 215 degrees.
colder than 195 and the fuel eccon will be low.
above 220 either there is low water flow ( plugged Radiator) or poor air flow accross radiator.
At 260 you are too hot.
a stuck closed thermostst will run hot
a stuck open thermostat will run cold at high speed (50 mph)

Have you checked the thermostat? Over heating will damage engine, do not drive car untill you fix over heating you will kill engine!!

yeah it sounds like it is sticking,,,i know some of the GMs have aoring seal in the thermostat and over time it gets stuck inside the thermostat and causes weird issues

Hi and welcome to FixYa,

Offhand, your described condition indicates a sticking thermostat, that is there is considerable delay before the thermostat opens. It is also possible that there is some restriction wherein the thermostat is not opening completely when the triggering temperature has been reached. As a test and to confirm, you could try temporarily removing the thermostat and operate the engine without one; observe temperature. The side effect of this test is that it would take some time to warm-up the engine, but operationally should not cause anything untoward. Of course, care has to be exercised that the gasket would still be intact.

Good luck and thank you for using FixYa.