Are you creating the picture and want to make sure the text stays sharp? If so, save a new copy from the original image at a higher resolution/quality.
Or are you trying to take an existing blurry picture and make it more readable? If so, this usually can't be done since the detail is already lost. Even though they 'magically' do it on CSI, once the detail is gone from a picture there really isn't any way to get it back.