Use the scale image selection in the menus (I don't recall where on the menus it is precisely, but it's there). You can scale larger or smaller. Remember, the larger you make the image, the lower the quality.
Also, when you scale it, if you don't scale both dimensions (h and w) byt the same ratio, your image is going to look stretched. GIMP scales with ratios, so the image doesn't get stretched. You can do one dimension or the other, but just realize what this might do.
Also work on a copy of the image, or save the altered image with a different name. Always keep the original image intact.
