If you don't need very accurate dimensions, say 3mm instead of 3.12mm, then click on the dimension, and change the value after the decimal point, in this case to zero. The dimension then will read 3mm.
If you want to fudge the dimension, like putting 7mm instead of the actual 3.12mm, then click on the dimension and you will be given a window with 3.12 written in it. Change the value to 7mm. This forces 7mm to be displayed, but it will have underline bar to indicate that the dimension is not true.
Best practice is to go back to the solid part and update the dimension to what it really needs to be.