1) Open the programme.
2) Click File, Open, find the photo you wish you change, click Open. (You should now be looking at your photo)
3) Click Image, the click Resize.
4) Under the Print Size header, change Units from 120 to 300.
5) Make sure that Keep ratio is checked, and make sure that Resample is unchecked. Click OK.
6) The photo should not look any different, but the DPI has been changed.
7) Click File, the Save As (usually wise to leave the original unaltered).
8) Repeat 2 - 7 until done.