Pressing the Option key while viewing a menu reveals hidden menu items. Shift-Command-4 takes a picture of the part of the screen you select. Shift-Command-3 takes a picture of the entire screen. In some apps, controls whether pressing the arrow keys moves the pointer or scrolls in a window. (You may need to press and hold the Fn key first) In some apps, controls whether pressing keys in the numeric keypad enters numbers or moves the pointer. Press Fn–Left Arrow (Home) to scroll to the beginning of the current document press Fn–Right Arrow (End) to scroll to the end of the document. On the Mac, press Command-Left Arrow to move the insertion point to the beginning of the current line press Command-Right Arrow to move the insertion point to the end of the line.
For example, pressing Command-S usually saves a document or file.
Change the picture for you or others in apps.
Install and reinstall apps from the App Store.