OK so we are looking at a tire diameter change, (29 or 32 or 40?) it doesn't really matter...
You can reprogram as stated above, or do it the way I do....
Put the new wheels & tires on and have a friend with a cell phone follow you. They can then make sure you are traveling a specific speed. Test it on the highway and in the city at slower speeds.
Now it's just math.
The speedo is "X" m.p.h. off.
Not as precise as the above but will give you real world accuracy.
I have mechanical Aptitude.
I can screw up anything.