Sigma BC 500 bike computer "SET MPH"
Changing the battery has very likely reset the memory of the bike computer, so it needs to know what units you want to use, and what tyre size you have on the bike etc. You can even tell it what your old mileage total was so it starts counting from where you left off. I guess that’s how the world managed before Strava ;)

If you still have the manual you should refer to the setup process in it. 

If you don’t have the manual, there are copies online you should be able to search for, as well as videos showing how it can be done. Whilst the video I watched was informative and matched the manual for a bc500, it sounds a bit different to what you are seeing, but the process will be very similar. 

This video might be helpful: