[chirp_users] programming

I have a basic programming question maybe someone knows the answer---As an example--I downloaded from repeater book lets say 10 channels.

Several days later I activate Chrip again to add more frequencies via repeater book I did thew\ download and find out that the first ten channels data has disappeared and replaced by the second set of frequencies. My question:  How do I prevent that from happening?       73' Bob WB3DYE