Changing from A - D on a run

Please don’t shoot me down on this one as its probably in the Forum staring right at me but I can’t bloody find it

Joining a group run I was put in Group A for the workout but needed to be in Group C/D
I was told to hit the box top left but nothing happened

How do you change to be in your correct group

I can hear you all moaning now but please stick with me I am a simple soul