I have two screws I cannot get out because the 6mm hex pattern on the head of the bolt is worn out in a circular motion.

Basically the hex is worn.

How do I remove screws/bolts? What tool or tools do I need...

 

Depending on where they are you can use a screw extractor--they reverse thread in,or needle nose vice grips to grab the head. Some people dremel the head to either cut a slot for a screwdriver or cut the head off and use a vice grips to twist out what is left.

 

One is right up against the heat shield holding up the trans dipstick mount and the other is on the cold start valve mount on top of the intake manifold under a bunch of lines...

Im thinkin grind the one on the trans dipstick and get an extractor for the other one because of its location...

 

Another option is to JB weld a nut on the end and use that to extract the screw--assuming you can get in there to weld it on. If you have rounded the hex a torx may fit in just tight enough to turn.

 

I got them out. Used a grinder attachment for my electric drill which cost 5 bucks. Cut a few notches into that bolt on the Trans dipstick mount and it came right out.

As for the screw on the cold start valve all I did was drill a shallow hole into the center which gave me more leverage and it came out...

 

Hammer a slightly oversize Torx bit in there... 95% of the time works for me!
Make sure its a good quality one too...