Video is located here
http://www.youtube.com/watch?v=nwgyQjenz1o&feature=player_embedded
1) Wait for your stats on any of your turrets to hit 5 Maximum.
2) Pause the game.
3) Hit the tab button until you get three plus (+) sign boxes (these are for upgrading a stat) that are highlighted.
4) Max sure your upgrade button belongs to a max stat.
5) Now when you have the upgrade plus symbol highlighted with a yellow box hit the space bar/
6) If you did this correctly the term max will change to one of the following [i] [NN%] [0]
7) Your cash value will change to NN Which is invincibility.

Make sure you keep pressing tab as a trick to make sure one of those are highlighted.
9) Once you have the part that says [i] highlighted press tab once and you’ll have damage highlighted.
10) Press it again and you will have the fire rate and target highlighted.
11) If you highlight it the 3rd time then the range is highlighted.