I have a refrigerator with 1 3 7 9 for temperature control. Where should I keep the indicator to keep it coldest? The milk is getting sour.
The bigger number is probably (but not necessarily) colder; as others have said the manual is the easiest way to find out.
Of course, if your only problem is the milk going off too fast, and it's a good temperature for the rest of the stuff, you might want to just put the milk in a colder part of the fridge. Near the vents is much colder than the door, and the back is usually pretty cold too.
Read the manual.
If you don't have a manual, buy an inexpensive refrigerator thermometer. Measure the temperature, then adjust by about, say, two settings, and 24 hours later, read the thermometer again. Whichever direction it changed, you will know which way the settings go. You will then know which way to adjust, and be able to target your ideal temperature.