when placing a power/GND port in EEschema I click on the 'ground symbol in the menu right on the screen.(when hovering over it, it says “place power port”). This opens up the part manager and shows the KiCad Power Library.
How do I reroute the path of this button to my own power library?
KiCad 5 should show any library that has at least one symbol in it that is defined as a power symbol. Details see Tutorial: How to make a symbol (KiCad v5.1.x) (Section Creating a custom power port (power symbol or power label)