Place the "STANDARD" in the micrometer and measure it (your own "FEEL" is very critical here). To adjust, use the spanner wrench and rotate the "TUBE" that is inside the spindle. This would be the tube that does not normally move, but it can be rotated with the spanner. You will find a small hole at the base where it buts up to the body of the micrometer.
Go to MITUTOYO.COM and look at there product breakdown (parts list) to see how the micrometers are put together.
This is all basic stuff, not hi-tech at all.
Hope this helps,