Some rectangular chandeliers are easy to adjust with a ladder and a little planning. Not all chandeliers are designed to have an adjustable height, so you might have to cut the chain or downrod to size or work with a professional to modify it.
Some chandeliers include multiple downrods of different lengths that you can unscrew and swap out as needed. Others use sections of chain that you can hook into place at the desired height. Both of these require you to remove the chandelier from the ceiling temporarily for adjustments, but this is relatively straightforward, depending on the weight of your rectangular chandelier.
Some chandeliers are cumbersome and challenging to adjust, especially if your home has high ceilings. In this case, you might want to hire a professional to help.