On the window that pops up you may choose the period for which you want to amend rates, the room types that will be affected, the percentage (or amount) that you want to add or remove and you should also choose if you want to round the results as well as which pricelists should be affected.
If you want to increase the rates you just need to add the extra percentage or amount (e.g. 20% or 20 EUR). If you prefer to decrease the rate you must add the minus (-) sign as well (e.g. -20% or -20 EUR).