The radio frequency of license free radio telemetry modules is pre fixed by the DTI, FCC regulations etc, you cannot change them.
What you do is personalise, or set an address if you like, on your receiver end, so that only your unique address, as sent from the transmitter end, is responded to. Let's say you sent out a 16 bit data burst of binary data from your PIC controller to the data pin on your transmitter module of
10101010 01010101 (from transmitter) (AA 55 hex)
plus the actual data you want to send, say a byte value of 41 hex (65 decimal) or (01000001 binary) or ascii character "A"
then your receiver end will see 10101010 01010101 (AA 55 hex) plus 01000001 (41 hex) (your data)
you set your code in your PIC at the receiver end to only respond when it receives the correct address, your unique binary pattern of
10101010 01010101, unless it gets this first, it just ignores whatever follows...if on the other hand it receives it, then you know that whatever follows is actually your data...