I just bought a second watch. This asked my to put on manually how many kms I made in the machine and I put a wrong number. This may affect the efficiency of my watch to track my next races or trainings? How can I fix this number?

It will affect the next treadmill run since you calibrated it with the wrong value. Just calibrate it again the next time on the treadmill. You must pretty much calibrate it every time anyway. 

Thanks a lot for your answer. Now, how can I calibrate it? The first time it just appear. It is a way to do it manually?

You will have to run treadmill again and the watch must measure at least 1 mile. Then you can select calibrate and save.