Highlight the Base Price of the item and type the new price, then click OK to save. 

If you need the price of the item to change by time-of-day and day of week (like for Happy Hour pricing) put the lower price in on of the prices 1 - 16 beneath Base, Alt and Sale Prices. Then check the weekday boxes you want the price to change to affect and set the From time on that line to the start time of the change and the To time to the time the special pricing ends. Any time outside of the t-o-d d-o-w pricing the price will default to the Base Price.