Changing the LIS3MDL magnetometer’s resolution requires writing to its registers, specifically, CTRL_REG2. If you are using our LIS3MDL Arduino library, then you can use the writeReg() function for this, and you can see examples of how that function is used in the definition of the enableDefault() function from the LIS3MDL.cpp file. You can find more information about each of the LIS3MDL’s registers in the “Registers description” section of its datasheet, which is available on the product page under the “Resources” tab.
As for the values from your magnetometer, can you provide more details about how you are using the magnetometer and what sorts of values you are expecting from it? For example, are you just looking at the output from one of our unmodified example programs, and have you done anything to try and calibrate your magnetometer yet?
In case you are not thinking about it already, keep in mind that it is especially important to calibrate magnetometers because their readings can be distorted enough to make their uncalibrated measurements more or less useless. Here are some links to threads with discussions about how you could go about calibrating magnetometers, though please note that they do not all refer the same hardware, programs, or libraries:
