Identify and select the product whose price you wish to match with a competitor's.
Expand the competitor's list by clicking on the cross symbol on the left side of the product listing.
Select the competitor whose price you want to match by clicking 'Match this':

You may choose to change the price, mark whether it should be your standard price or your offer price that needs to be changed, and press 'OK'. Then the price will automatically change on the product in your shop.