Cycle back and forth between the entries of the abilities below the Score column. Most web browsers support this by interpreting the TAB-key (with or without the combination of SHIFT).
Entering and changing the score automatically updates the respective ability's costs, the total amount of points being spent so far as well as the amount of points remaining. It also calculates and displays the ability modifiers.
Don't try to change the entries of the Modifier- or Costs columns. They are read-only fields and updated along with the entered ability score.