NEED HELP TO CHANGE FREQUENCY TO GARMIN T 5 COLLAR TO USE IN

Dear Friends
Last year I lost my Garmin Alpha 50 and left only with a T5 which work in 255.600 MHz
I bought a Garmin Alpha 50 from Sweden which works (in 255.450 OR 255.475MHz) just fine with its own collar T5 but cannot pair with old collar
How I can change my old collar frequency to work with my new device?

Thanks in Advance