how to get rid the sourness in yogurt
If you want less sour yogurt, you have to pick the right culture and right process. 


First, choose a streptococcus culture, or maybe bifida. Lactobacilicus bulgaricus gives you more sour yogurt. 
Second, go as low as your culture allows you. The manufacturer will have given you the range at which your culture can be incubated, choose something at the lower end. But of course make sure that whatever setup you are using can really hold the temperature in range - for example, if you have a culture that can go from 40 to 46 Celsius, but your apparatus has temperature swings of 1.5 degrees Celsius, don't set it to 40, set it to 42. 
Third, time. Here, you have to see what is the shortest time you can incubate and have the yogurt set. It is a matter of trial and error with a given temperature and culture. Moscafj's answer suggests 5 hours as a starting point for experimentation. 
Fourth, storage time. Your yogurt can keep going slightly more sour in the fridge. So don't make more than one week's worth ahead. 
Fifth, make sure that your starting temperature is also correct. I have a relative who is too impatient to wait for the yogurt cool down properly before innoculating, and never uses a thermometer, she just adds the culture to the too hot milk. Her yogurt always reeks of acetic acid to levels I can't tolerate. 


If you get your variables right as described above, your yogurt can be pretty mild.