Open it in an image editing program (MS Paint, Photoshop, GIMP..) and depending on the program there will usually be an 'Image' tab at the top of the screen, click it and there should be an option to resize. Then just enter the size you want. If your making something bigger though it will lose quality.
If you don't have Photoshop I'd reccommend using GIMP
(free) to resize as it uses filters and does a much better job at keeping image quality. Resizing in MS Paint.
There are also online sites which will do all the work for you but I don't know any names.