Chrysler Repair: Thermostat, inlet hose, thermostat

Question
I was wondering how hard is it to change thermostat on 1999 Sebring 2.5. The reason is I've never changed thermostat in 10 years & the car warms up but the heat gauge will take around 25 minutes just sitting in driveway when it's around 5 above so my thought is the the thermostat is some what open which make the car get really bad gas mpg but it does warm up when driving. The car allways got around 23 mpg or so in winter but now it get around 12 mpg any thoughts would be helpful.  Thanks

Answer
THERMOSTAT - 2.5L
REMOVAL
1.Drain cooling system to the thermostat level or below.
2.Remove inlet hose and bolts from coolant elbow. Remove elbow. Thermostat, Housing and Inlet Elbow
3.Remove thermostat assembly, and clean sealing surfaces
INSTALLATION
1.Install thermostat into the recess in the thermostat housing. Install inlet elbow and tighten the bolts to 12 N·m (133 in. lbs.) Thermostat, Housing and Inlet Elbow
2.Fill cooling system. Refer to Refilling System outlined in this section.