You can change the order of your categories or the videos in the categories at any time. This can be useful to bring relevant content to the front, move older videos to the back, or make related content appear side by side.
To change the order of your categories, go to MEDIA - MEDIA LIBRARIES in your media library and select MANAGE ELEMENTS. Here you can see the categories of your media library in chronological order. The category above is the first category and the category below is the last category. You can now drag and drop them as you wish. After that, be sure to click UPDATE SORTING to save the changes.
For changing the order of the videos, you already are in the right place, now you just need to click on MANAGE ELEMENTS at the category where you want to rearrange the videos. Now you will see the videos arranged chronologically from top left to bottom right. You can now drag and drop them as you like. Afterwards you have to press UPDATE SORTING again to save.