When the prompt to switch appears at the relevant depth you need to make sure that you have the gas bottle showing in the bottom left hand corner of the screen. If it says Dvo instead then short press left to bring back the gas bottle symbol. (Short pressing left just cycles between Dvo and the gas bottle symbol)
So with the gas bottle showing long press left which will then take you into the confirm screen. Assuming it is the gas you want (you did set them before the dive after all !!) then you will need to short press right (or long it wont matter) and prompted by the 'tick' symbol in the bottom right hand corner of the screen.
You will now be returned to the main screen and within a split second the new gas will be registered and the prompt to switch will have disappeared.
Hope that helps.
Graham
.