Layers are a powerful feature in image editing software that let you create certain special effects. Photoshop supports the layers feature, and another program that also supports it is the GIMP, a free, open-source graphics software. You can open a PSD file that contains layers and edit it in the GIMP, just as you would edit a regular GIMP file.
Open the GIMP and navigate to the location of the PSD file on your computer and open it. Or locate the file in your file manager, right-click on it and select "Open with the GIMP" from the menu that pops up.
Select "Windows" from the top menu bar, then "Dockable Dialogs" underneath that, and "Layers" from the window that pops to the side. The Layers dialog is on on the top of the floating panel that generally appears on the right if you have not moved it.Click on a layer in the Layer dialog
Click on a layer in the layer dialog to select it. Then you can edit that layer by using the tools in the toolbar or right click on the layer's name and select what you want to change from the menu that pops up. For instance, you can change the name of the layer, or use "scale layer" option to change its size. You can use the Move tool to move it around.
Save the file as a PSD file (over top of the original) or an XCF file, GIMP's native file format, if you want to preserve the layers and be able to edit them in the future. Then use the Save As option to save a Jpeg or other image file for use in a document or web page.