1. To merge two adjacent layers, go to the “Layers” menu, select “Merge” and then click on “Merge Down”
2. To merge all layers in a group, go to the “Layers” menu, select “Merge” and then click on "Merge Group"
3. To merge all layers, go to the “Layers” menu, select “Merge” and then click on "Merge All".
4. To merge all visible layers, go to the “Layers” menu, select “Merge” and then click on "Merge Visible"
Note that when layers are merged, they can no longer be edited separately.