Go into Club/Stadium Facilities/Stadium Expansion. Change the stadium type (rounded corners, oval; this will cost you money) and select "Build Stadium". Do this again (changing the stadium type, build it), and you will receive money. Note: Do not do this if you are thinking about building a new stadium.
Always check the player status screen before you sell a player. If a player has a close friend on the team, this will cost a lot of morale. This could be disastrous if the remaining player is one of your stars.