How can I change the DPI attributes of a JPEG file?

Situation: I have a JPEG file with an invalid DPI setting. Preview shows a DPI setting of 1 for both dimensions. Problem: When trying to print such a JPEG file, it fills a whole DIN A4 page even at 1% printing size. The print dialog does not support fractional size percentages, so the image cannot be sensibly scaled when printing. Solution attempts: Changing the DPI setting of the JPEG file to a value >= 100 DPI will fix this. The actual value does not even matter much. Any value >= 100 will do. Remaining question: How can I change the DPI attributes of a JPEG file?