The hardness number is the one setting on a softener that tells the control valve how much hard water it is treating. It drives how often the unit regenerates and how much salt it uses. Set it too low and hard water gets through to the fixtures. Set it too high and you pay for salt and water the house does not need. The question is not whether you can change the number, since many units let you. The question is whether you have a trustworthy reading to change it to, and whether the control is simple enough that a mistake is easy to catch.
Start with the reading, not the menu
Before touching the control, find out what your water actually measures. A current test from a lab or a reliable test kit gives you a hardness figure. Check the units on the report. Hardness is commonly listed in grains per gallon (gpg) or parts per million (ppm), and the control may ask for one or the other. One grain per gallon is about 17.1 ppm, so a reading in ppm must be converted before you enter it. A test that is old, taken from a different tap, or taken after a softener was already running is not a sound basis for the setting. Write down where and when the sample was taken, then decide.
Check the manual and the control before you decide
Look up your model's manual and find the section on hardness setting. Confirm three things: whether the number is entered in gpg or ppm, whether the control has a locked or protected menu, and whether the manual says what the unit does with a changed value. Some controls ask for a reserve or safety margin on top of the measured figure. Others calculate regeneration from capacity and the number you enter. If the manual is missing, the label on the tank or valve usually carries the model and serial number, which you can use to find the right document.
When doing it yourself makes sense
Changing the number yourself is reasonable when all of these are true: you have a current reading in the same units the control uses, the menu is clear in the manual, and you can write down the old value before you change it. Record the new setting, the reading you used, and the day you made the change, then keep the note with the install paperwork. After the change, watch the unit through its next regeneration and check the water at the tap. If the water still spots or feels hard after a few days, revisit the number, but do not guess higher without a new reading.
When to hire it out
Call the installer or a service provider when any of these apply: the reading conflicts with other results or you only have an old one, the menu is locked or you are not sure which value is right, the water has other problems such as iron staining that a hardness number alone will not fix, or the unit is under a warranty that depends on who adjusts the control. A technician can test the water on site, set the number, and show you the new value on the unit. Ask them to write the setting and the reading on a card and keep it on or near the control.
What to ask before you accept the setting
Whatever route you take, ask a few plain questions. What reading produced the number, and in which units? Does the control add a margin, and how large is it? Will the setting need to change if the water source changes, such as a new well pump or a switch in supply? Who should you call if hardness returns at the tap, and what should you have ready when you call? A good answer is specific and written down. A vague answer such as 'it is set for your water' is a reason to ask again.
For a broader look at sizing and setting a softener, see the Missouri Water Softener Buyer's Guide.