The Following screen allows you to sub-select using various selection options for which products you wish to apply changes to.  On this window you can individually change item name, volume pricing discount schedule, product cost and base unit price to the products that meet the selection criteria at the top, or you can click on Mass Price/Cost Adjustment to make changes to all products selected on this screen:

Simply specify the increase you wish to do for cost and/or retail price and whether you want it to be a percentage or flat dollar amount.  Also specify how you would like rounding to occur.  Click on Proceed with Price/Cost Adjustment to apply your changes.

In the same manner if you want to do a decrease, simply use negative numbers.  So instead of a 10% increase, you can do a -10% decrease.