There are a few different ways to do it. The first is to click on the ESP whose timestamp you want to change. Then, in the Modified area of the column on the right, type in the new date and time you want. Click save down on the bottom. You're done.
Another is to right-click on the column head above the mods, to bring up a menu. Choose Sort List, and then, Load Order. Click on the file you want to move. Hold down CTRL, and use the arrow keys on your keyboard to move ESPs up or down.