The microscope eyepiece diopter is adjusted by rotating the top of the eyepiece. Microscope eyepieces with diopters have numbers or marks on them in order to easily adjust eyepiece settings for different users.
How to Properly Adjust Microscope Eyepiece Diopters
The way to correctly adjust a microscope eyepiece diopter is to first close the eye over the eyepiece with the diopter adjustment and normally focus the microscope so that the open eye sees the image in focus. Next, switch eyes (close the open eye and open the closed eye) and without changing the main focus knobs, focus on the image by turning the diopter adjustment only. Now with both eyes open, the microscope image should be clear with both eyes. (This technique is also used when using binoculars.)