It is easy to change the logo. Just name a same name as the present logo and upload the same directory of the theme of the image sets of the forum and reload it.
Normally PhpBB selected prosilver theme. Upload the uploaded logo in styles\prosilver\imageset then reload it.
We can also change logo from admin.
Just upload the image in styles\prosilver\imageset then go to your admin panel in Style ->Imagesetes->prosilver and then edit
then you see that the Image and select the desire image.