Ownership of files can be transferred. Once users transfer their ownership their cannot transfer it back to themselves unless the new owner changes the settings. After sharing the file to change ownership follow the steps given below.
OpenShare with others pop-up menu. This process is the same as sharing a file.
Click on Advanced on the bottom right
Sharing Settings will pop-up.
On the right side of the people with whom the file has already been shared click on the Edit Icon/Down Arrow.