You Can Choose From Two Things:
1- Increase The Time.
2- Finish The Level Faster.

Open The Game Root Usually: C:Program FilesSmall RocketsNFK - Natural Fawn Killers

Go To The Directory Levels
In That Directory You Will Find The Levels And They Are Sorted As Follows:

Level 1 CORPSES (First Level).
Level 2 RESERVR
Level 3 WABBIT
Level 4 BEARNESS
Level 5 ONEDEER
Level 6 MIDNIGHT
Level 7 BREEDING
Level 8 HEARTBLD
Level 9 BUBBA
Level 10 SPRING
Level 11 LASTWRLD (Last Level The T-Rex)

Now go to any level you wish to modify and open the file info.txt. If you wish to increase the Level Time, just increase the number next to time.

When you are finished modifying the level just save it and play.

NOTE: ALWAYS MAKE A BACKUP BEFORE OVERWRITING GAME FILES!