Tap the blue menu button (next to the search bar in your Diary). Select 'Week View' and use the left and right arrows at the top of the screen to move forwards and backwards one week at a time.
On the website
You will find 'Week View' at the top of the food diary page. Clicking on the arrows to the left and right allow you to move forwards and backwards one week at a time.
Or to move back several weeks, use the calendar icon just above the Add Food panel you will see the date of the day you are on. To the right of this is a small icon which is a calendar. Click on it to bring up a pop up. Then you can pick any date you want and click on it to move back or forward.