To close (also known as exit) MS Excel

Click on the  Office button on the  menu
Click on Close

or

Click on the X in the the top right hand corner of the screen.

Note: You should normally save your work to disk before closing a program. If you do not do this, you will lose your work. If you have accidentally forgotten to save your work, Excel will notice this and you will be asked whether you want to save it. the following pop-up screen appears.

Click on Yes if you want to save the work. A save pop-up screen appears.
If you do not want to save the work, click on No.
If you do not want to close (exit) Excel, click on Cancel.