Note: This drastically alters a system file. Back it up before you muck around the file, or you'll be sorry. E-mailing IGN because you mistakenly alter your FAT is laughable.

Go to the Tankcommander/.../data/profile/save/ game directory and locate the checkpoints.txt file using a text editor. In that file there is a line value:

Level 1-Complete 08/24/05 none

Copy that line and paste it in the same file. Then change the level number and save the file (as text).