Assuming it's a standard setup...
1. Unplug
2. Turn off the cold water feed to the dishwasher.
3. Disconnect the water feed, and drain hose.
4. Remove the kick plate if there is one
5. If the washer is screwed to the counter, unscrew it
At this point you should be able to slide the unit out.
You should have installation instructions with the new machine, but basically you...
1. Position the new unit in front of the counter
2. Run the power cord, drain hose and water feed through the cutouts in the cabinets...connect.
3. Connect the drain and water feed to the washer. Use teflon tape on the brass connections.
4. Measure the floor for level, and adjust the leveling feet as you see necessary.
5. carefully slide the washer in, and check to make sure it's level and the hoses are not pinched, readjust as necessary.
6. Check to make sure the door has clearance
7. Screw into the counter, assuming it has mounting brackets there.
8. Install kick plate
9. Turn on water, plug in and test.
That's from memory at least, someone please correct if I flubbed anything
-spence