In Altium Designer how do I navigate to the component editor from the PCB layout screen?
You can't, so far as I know (and I know alot about Altium), edit a component in any way by just clicking on an instance of where it was placed.

If you want to edit a part, you have to edit it in the library.  You can then do an "update components from libraries", or just delete it and drop down the new version from your newly edited library.

You can, if the part has it, drop it from a library and then select from previously defined footprints.  Like, you could have a resistor that has 0402, 0603, 0805 etc  footprints associated with it.  Drop the part, choose the footprint.  But you couldn't drop the part, click on it, then go edit the schematic symbol or tweak the footprint. You have to do that in the library.