Whats going on with 1999 Pontiac Sunfire?

Recently, like within the last month, we replaced the water pump on our 1999 Pontiac Sunfire. Now, the temperature gauge won't go into the normal operating range, it stays below it, and the heat blows out lukewarm. Any suggestions?
ok, how hard is it to replace the thermostat??? and where is it at?
 
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