How to chang a themostat in a Kia spetra 2003

I am trying to change the themostat in my 2003 themostat Kia Spetra. Or seal the gap. I tried to just seal it due to my boyfriend is away. But notice he said have to take out alot of parts to get to it. If cant get replace what can I use to get the sealant that is on to put fresh sealant andwhat can I use to get under the area the needs to be sealed. Water is just running out underneath.