Open Photoshop
Go to File > Open and select both photos
Choose the Move Tool
Drag one photo onto the other document
Use Ctrl+T or Cmd+T to resize and position the layer
Add a layer mask to the top photo
Select the Brush Tool
Paint on the mask with black to hide parts of the top photo
Paint with white to reveal parts of the top photo
Adjust opacity if needed
Use the Eraser Tool only if necessary
Go to File > Save As and save the merged image