How to use a PSD mockup in Photoshop
A mockup is a template for previewing your design on an object or screen. These steps apply to PSD templates with Smart Objects.

Open the mockup
Download and unzip the product. Open the PSD file in Photoshop and find the Smart Object layer with the placeholder artwork in the Layers panel.

Replace the artwork
Double-click the Smart Object thumbnail. In the document that opens, delete the placeholder, insert your artwork and scale it to fit. Save and close it.

Save and export
Back in the main file, save your working PSD. Then choose File, Export, Export As, pick JPG or PNG and export the finished image.
No Smart Object layer?
Some templates are built differently. Follow the instructions included with that product, or pick a mockup that lists Smart Objects in its features.