To be able to edit an animated gif in Photoshop, you will need the original .psd file. If you try opening an existing .gif you will find that you can only view 1 frame of the animation.
Open the psd file and open the animation panel if is not already open. To do this go to Window > Animation.
You will either be presented with the Animation Timeline or Animation Frames.
Click on to Animation Frames by clicking on the symbol in the bottom right corner of the panel to show the Animation Frames.
Under each frame is the time the frame will be shown for in the animation. In this case 0.2 seconds.
To speed up or slow down the animation click the time shown under each frame and change the amount of time.
There are preselected amounts of time shown in the menu on click. To set your own value. Choose ‘other’ from the list and you will get a pop up box where you can manually enter the time that you want.
If you are interested in learning more about animation in Photoshop see How to create a repeating circle animation in Photoshop.