First, you have to have permissions. If you don't this won't work and you need to find someone who does or get a manager to get you permissioned.

Secondly, this gets logged, don't abuse it.

Scan the item you want to change the price of and press F2. (If you don't have those permissions this won't work.) If you want to straight change the price, just plain change the price-- it's self-evident how to do this. (With some items you'll have to page down to the second or third page and change those, too, but that's rare.) Use F5 to save.

However, you have another option: leave the full price as is and change the plus card price. This is generally recommended for in-store specials, because it shows up as savings on the receipt. To do that, you go down to the "coupon" section at the bottom right corner of the first page and put in the correct coupon code for the price you want. For items priced per pound you use the code 28xxx, with xxx being the price per pound (ex. 28199 for $1.99/lb.) (There are codes for over $9.99 but I don't know them offhand; there should be a cheat sheet somewhere on the RDIC website, or poke your pricing person for help.) For items that are going to be a set cost of $8.99 or below, use 31xxx, with the xxx being the price (31349 is $3.49 each.) The limit is $8.99 because 319xx is special-- it codes for a % off (so 31925 is 25% off, not $9.25.) Codes for sales on items that cost over $8.99 are also on the cheat sheet. 

There is no way to add or remove items from Buy X Save Y, triple fuel point, and similar promotions locally that I'm aware of. 

If you're trying to match an item that's on sale (for example, if all the flavors but one ring up on sale) copy the coupon number from the item that works.