Page-Up and Page-Down Keys
Press the page-up key to increase the value of a cell by 1 and Page-down to decrease the value by 1.
You can also select a range of cells and increase or decease them by pressing the Page-up and Page-down keys. Use Shift and any combination of arrow buttons to highlight / Select a range of grid cells that you want to adjust and then press the Page-up or Page-down key to increase or decrease each cell by 1.