In the second tab of the Anvil interface, you can re-forge your gear. For some reason, I cannot re-forge anything. I am either unsure how this works, It is purposely prevented, or it is a bug.( example:

)

It still doesn't work.(sorry it took so long i went to bed)

It would seem that it's not possible to reforge paxels, I'll do some digging to find out more.

make sure items are fully repaired, you cannot reforge something that is damaged

Even spawning in a brand new paxel doesnt allow it to be modified by Tiered.

It would seem the Tiered mod does not see the paxel as a item that's a valid option, as other items you can shift+click into the anvil GUI, Paxels you cannot.

edit

Modifiying the NBT data of a paxel to give it a tier also crashes the game when Tiered tries to render the tooltip, sadge.

Reforging also requires amethyst in the 3rd slot

gotcha.