Assuming you want to remove the entire roof:
First, you'll need to remove any bolts or screws that are holding the roof in place. Once these are removed, you should be able to gently pull the roof off of the car. If it is stuck in place, you may need to use a pry bar or other tools to loosen it up. Be careful not to damage the car while doing this.
Once the roof is loose, you can carefully lower it down and set it aside. You may need help from another person to do this safely.