Being а ruler of your fortress, you can move your buildings at the map wherever you wish.
To do that you should first of all enter the Edit Mode. Press the Spanner button at the upper panel, or press E on your keyboard.
After that you will be able to move your buildings at the map.
Every building occupies certain number of squares. As you can see, the squares are limited, but don't worry - all your constructions will go in.
You can move a single building by dragging and dropping it to a new spot, or you can put all your buildings to internal storage and rebuild your town from scratch.
Just press Clear the Map button.
All the buildings will be placed to a storage and the city map will become empty. You should place the buildings one by one on the map again.
You can either Save your new map when all buildings are placed or Cancel the changes at any time.
You can't delete any of your constructions, but you will never need to - all buildings are useful in the game process!