There's an ending guide here: https://desidee.itch.io/changing-time-vr/devlog/754383/ending-list
It's obvious, but warning regardless: spoilers!
Fully charge both the green and blue gems from the exit altar, and place them back in their original positions (on the left and right sides of the altar). Use the buttons on the back of the mirror (eye, sitting person, owl, hawk) to unlock the pyramid. Take the gem from the pyramid, fully charge it, then place it in the center of the exit altar. Hope that works for you!