This is very similar to a magic square. Here though there are no diagonals, so it is just the two rows and two columns (shown in grey text)that must add to the same amount.
Click on a number to select it, then click another to swap them over.
Hint: How many totals is each number used in? Any that are counted twice will increase the grand total of the columns and rows above the simple sum of numbers 1-9.