Open the image in a photo editing program such as Photoshop, or Gimp.
Select Image or Image Properties.
Select from this menu the Image Size option
Where it tell you the dpi value, simply increase this number, for example from the native 180dpi to say 400. You must make sure the resampling options are set to the optimal for enlargement. The software will resample and recalculate the pixels surround those captured.
When applied carefully, the results can be made to be much more useful than you may have thought.