Jewelry does not change the appearance of your Player Character. Instead, Jewelry is used to hold additional Item Sigils. How many Sigil Slots a piece of Jewelry has available depends on the type and rarity of the piece. For more information refer to the Forge Sigil section in this manual.
To equip Jewelry press I to open the Inventory and press O to open your Equipment. Now click and drag any Item of Jewellery from your Inventory to the associated highlighted Slot in your Equipment and release. Your Player Character will immediately have the Item equipped. Reverse the actions to unequip it.
Note that to change Jewelry, you will need to be Out-of-Combat.