How do i link w/sheets to increase value without just overwriting
 

I am just learning to use excel and i need help. I have written a purchase
order form, then on individual tas inlcuded all relevant information about
staff members. I want to keep track on uniform expenditure. So i want to be
able to change the info in the purchase order, and hopefully this will update
on each individual worksheet, keeping a running total instead of just
overwritting the previous value. Thus keeping a running tally of spending.
Any help will be greatly appreciated.