Select the layers you want to merge in the Layers panel
Right-click the selected layers
Click Merge Layers
Use Ctrl+E on Windows or Command+E on Mac to merge selected layers
To merge all visible layers, click Layer in the top menu
Click Merge Visible
Use Shift+Ctrl+E on Windows or Shift+Command+E on Mac to merge visible layers
To flatten the entire image, click Layer in the top menu
Click Flatten Image