If the slides you want to move around are next to each other, you can “drag and drop” them in the Normal View. Click on the small image of the slide in the left column, and drag it where you want it to be. When you see the horizontal line, let go, and the slides will change places.
If you have a lot of slides, it might be easier in the Slide Sorter View. Go to the View Ribbon and click on the Slide Sorter icon (or click on the icon in the Status Bar). This view allows you to view multiple slides at once; click, hold, and drag the slide over to where you want the slide positioned, and then let go of the mouse button.
To move multiple slides at the same time, hold down the Shift key as you click to select consecutive slides (Control if they are not consecutive), then drag them to the new location. You can also select slides and copy and paste them into a different presentation. Double-click on a slide or click the Normal icon to return to the Normal View.