1992 Other Mercedes Benz Models engine cooling

1992 Other Mercedes Benz Models 6 cyl 185000 miles

Simple problem I think, but want to ask anyway
Live in SO. Cal. Desert areas, temperature gets pretty
hot, when A/C engaged on my 1992 300e, if u are driving in city where speed is slower, temperature
starts moving up close to red area, should I replace thermostat with cooler one and if so how cool, Want
to keep engine as cool as possible, I have no idea what
temp thermostat in now, Someone please help me

THANKS
TODD