Manually editing the geotag isn't yet available in Google photos. Google photos in some cases will add a location tag automatically though based on your location history. If you want to manually edit the geotag, you could try these options:
Workaround: change it in Google+ photos, the changes will be reflected in Google photos as well:
- Go to https://plus.google.com/u/0/photos/yourphotos.
- Click on the picture/video you want to change the geotag of.
- At the right-hand side at the top click on "Details".
- In the pane on the right, you can change the geotag.