You can level up Relics by using Relic XP items.You can also break down unused Relics to acquire addition Relic XP Materials.

Each Relic has a main stat and a set of random substats which get unlocked and upgraded every few levels, so invest in Relics to see which substats each Relic has. Once you have Substats you like, you’ll want to select the Relics with the best stats and feed them Relics with less ideal stats. Each Primal has different stat goals and needs, so try and find the best Relics for each Primal!