Change wheels diameter from 28" to 26"
It can sometimes be done, but usually you will run into problems.

Major problems:


Brake compatibility: Rim brakes do not reach the rim in its new position. Disc, drum and coaster brakes are not affected.
Pedal clearance: Pedals will now be closer to ground and will hit the ground if you are not careful when turning.


Minor problems:


Steering will be different because the geometry changes.
Gearing will be lower because smaller wheel circumference.
If you are doing this so that you can have wider tires, they do not necessarily fit into your frame.
If you are doing this to have a smaller bike, it does not work. You still have the same size frame, just closer to ground.