Command Button to increase cell value

I'm having a bit of trouble with a forecast spreadsheet.

Basically I have a cell with a value that represents the absence % for my workforce, lots of other cells referrence this one to calculate the expected absence.

Now I could just retype the value everytime I want to adjust my planning, but really I want to add a command button to add .5 to the cells current value each time the button is clicked.

Any help?