Once you have finished editing the document, you can either close Office2 and your changes will be saved automatically, or you can press the Close button at the top left of the document window.
If you press the Close button and you have made changes since opening the document, Office2 will prompt you to save or discard your current changes.
Press the Save button to save the changes. The new document file will be shown in the files list. If you don't wish to save or discard changes, but instead wish to carry on editing the document without saving, press the Cancel button. Another way to save the document is by pressing the Save button located at the far left of the toolbar. When you press this you will be prompted with a Save and a Save As button. Choose the Save button. Any changes you have made will be saved and you will be returned back to the document ready to continue editing.
Office2 lets you easily revert back to a previously saved version of the file. For more information see Reverting to a previous version of a file.