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.