On this screen you add or remove initiative modifiers. There is a text box just above the done button, which allows you to input the value of the modifier. So, a value of 3 in this box with Increase Reaction selected would increase your PC/NPC reaction by 3.
Highlight the spell or effect you want then tap done to return to the Initiative Results screen.
Note: Values are not cumulative. Only the strongest effect is applied.
If you add “Increase Willpower 3” then add “Increase Willpower 1”, your PC/NPC will have the stronger “Increase Willpower 3”.