From the appropriate context (ie. from within the Site Visit record in Landscape if you're working on a normal monitoring visit) click on the 'layers' button on the map, select the three dots next to 'Photo Points' and click 'Edit Layer'
Click on the photo point you want to merge and drag it to the photo point you want to merge it with. The target photo point will turn yellow. Release the mouse button.
Click the thumbs up to save your edits.
Once you're no longer editing, you can click on the photo point to review it: