If the default grave accent key clashes with another mod or is awkward on your keyboard layout, you can rebind it in the vanilla controls menu.

1. Open the game menu and go to Options.

2. Open Controls, then Key Binds.

3. Scroll until you find the entry for the Elytra Chestplate Swapper (it appears alongside the other mod keybinds).

4. Click the current key and press the key you want to use instead.

5. Save and exit the menu.

Pick something you can reach without moving your hand off WASD, since you may need to swap while gliding.
Avoid keys already used for sprint, sneak, drop item, or your hotbar, or you will trigger two actions at once.
On AZERTY layouts the default maps to ², so if that is hard to reach, remap it to a side mouse button or a nearby letter.
If the key does nothing after rebinding, check for a conflict with another mod's keybind and change one of them.

You only need to set this once; the game remembers it between sessions.