um..you don't. It's a loop. It's in a loops nature to not change but be the same over and over again. That's why it's a loop.

Are you using some sort if MIDI device to play back this loop? If this is the case and you want to do something like change the chord from a D to E, then that can be done by changing the midi notes that are being played. But if you want the loop to play one chord then another you need to create a new loop with both chords.

Is the loop an audio recording? If that's the case then you are out of luck.

thanks bro brand new to loops so I am using apple loops , what if I change the key of the loop at the chord change?
not using midi, just loops from the hard drive