Locate the image that needs to be changed and click on it.
Locate the “Edit Image” button below the image on the Attachment Details screen and click it.
If you mouse over the buttons over the image, a tooltip will tell you which button does what. From left to right, they are: Crop, Rotate Counter-Clockwise, Rotate Clockwise, Flip Vertically, Flip Horizontally, Undo and Redo.
At the top right, you can adjust the scale of the image by typing in a number. It will keep the image dimensions as you change one image, because it is just scaling it down.
The current thumbnail image is shown here.
You must remember to make sure that you have selected the correct radio option for what you want your changes applied to. Default is all image sizes, but you can change just the image or just the thumbnail if you prefer.
Don’t forget to save your changes afterwards. Some save automatically, but not all of them.
