Smart Object > Replace Contents

Open the 'original' image and double click on the Smart Object layer. This opens it in a new window showing how it is made up.  i.e. Smart Objects generally consist of multiple layers.  You can see that you are in a new window as it will have a PSB format

Now select the Smart Object layer and do your Layer > Smart Object > Replace Contents

A load file window will open and you can select any image format (including layered PSD images) which will load into the Smart Object overwriting its original content.

Note this happens in a new document window.  If you use Ctrl s (Cmd s) that will save the change to the original document. You can then close the PDB file.

Does that help?

Can you post some screenshot with Layers panel visible?

Hi. What is your smart object made up of? If it's another external image, then perhaps you placed it using Place>Embedded? If so, you can convert it to linked, so externally editable and replaceable by clicking the Convert to Linked button in the Properties panel, when the layer is selected.