Typing over a formula

[LB_MJPA]

i have an excel grid with 3 tabs. Its a travel doc: Staff, Air, hotel. My associate put formulas in the first tab in order to tie all 3 tabs so I wouldn't have to enter people's info all 3 times. But as soon as i go to type in their info, it loses the formula. How do i type over a formula and have the formula apply?
sample; =+air!C3

 

[JackDanIce]

A cell can only contain a value or a formula but not both