For Windows 10

1) On the Start screen, select Store to open the Windows Store.

2) Select the account menu (next to the search box), then select Settings.

3) Under App updates, set Update apps automatically to Yes.

For Windows 8.1 and Windows RT 8.1  

1) On the Start screen, select Store to open the Windows Store. 

2) Swipe in from the right edge of the screen, and then tap Settings. (If you're using a mouse, point to the lower-right corner of the screen, move the mouse pointer up, and then click Settings.) 

3) Tap or click App updates. Make sure Automatically update my apps is set to Yes.