The same two letters, in the same order, will change each of the following words into new ones.

1. ELDER
2. RING
3. END
4. MIST
5. DEED
6. GOSH  (two words)

HINT:
The two letters precede words 1, 2, and 3, and are inserted into words 4 and 5.

Answers:

The letters are FI:

1. FIELDER
2. FIRING
3. FIEND
4. MISFIT
5. DEFIED
6. GO FISH 

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
FI UPON IT