When merging layers in Photoshop, you have several options. These options are found within the Layer menu on Photoshop’s main tool bar. However, the available choices may alter slightly, depending on the situation.
For instance, you may just want to merge a few layers together. One easy way to do this is to hold down the CTRL key and click on all of the layers you want to merge. When doing this, each of the selected layers will be highlighted in the Layers panel. Now, you can either select Merge Layers from the Layer menu or hit CTRL+E on your keyboard to transform these multiple layers into one.
Alternatively, maybe you want to merge everything except for a few layers. In this case, you can just hide the layers that you don’t want to merge, and choose the Merge Visible option.
Another common thing you may want to do is merge two adjacent layers together. As long as the bottom layer is a pixel-based (raster) layer, you can just select the top layer by itself and choose Merge Down from the Layer menu. If the bottom layer isn’t pixel-based, this option will not appear.
The most drastic option is to merge all of the layers together, so that the result is a single-layered image. If no layers are hidden, you can do this by choosing the Merge Visible option. You can also choose Flatten Image if you want your single resulting layer to also be transformed into a locked background layer. If you take this route, definitely save a copy of your project first.