I have a Mity 2. It may not be exactly the same as your Mity.
Find a button called "set button", mine happened to be the tiny rubber button between two large buttons. While function is set to Time, Average speed or Distance, each set button pushing would change auto to manual alternatively. You have to use some pointy object such as ball point pen.
From the manual:
• How to set ON or OFF the auto-mode
Push the SET button on the back side of the computer while Elapsed
Time, Average Speed, Trip Distance 1 or 2 is displayed, then the icon
is lit” (ON) or turned OFF.