When replacing the doors, the first thing you need to decide on the exact size of the door leaf. For this, it is necessary to measure the height and width of the opening. After the door is purchased, you can stop the dismantling of the old door box, unless of course there is a need for this. After removing the old box, it is necessary to remove everything that falls off from the walls. After that, using plaster and putty, it is necessary to close all the cracks and bumps found or appeared after dismantling the old box.