The graphic you attached is a flattened PNG. Do you have the original layered Illustrator file? If not, I'd recommend making the color change in Photoshop by masking out the mouse. As a raster graphic, it is going to be blurry at larger sizes. If you have to use a raster graphic, try to make it the largest size you need and scale down from there. If this image is all you have to work with, then you could try enlarging it and sharpening it in Photoshop, too. If you have questions on how to make the changes in Photoshop, you can post them in the Photoshop forum.
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