1. Select the objects you want to join. If these are adjacent or overlapped, a new larger merged polygon will be created. If the elements are not adjacent, a compound object will be generated.
2. Click on the Join Objects tool. A window will appear where you need to select the main object. The new object (merged or compound) will keep the data associated to the main object.
3. Click on Accept. The polygons will be joined keeping the data associated to the main object.