How to clean copper bracelets?

Copper is a red metal, also is a green metal. After a long time in the humid air, the copper surface will form a layer of patina slowly, this is oxidation. So if your bracelet turn green, it's normal and prove that the bracelet is made of pure copper.

Suggest you to take it off when you are swimming, taking a shower, or sweating for a long time, avoid the bracelet becoming green due to prolonged exposure to sea water, sweat or salt water. If it turn green, please don't worry,you could keep it shiny with below methods.

Cleaning Copper with Vinegar
Soak the bracelet in some vinegar for 5-10 minutes. Rub it clean if needed. Rinse thoroughly with water. Then dry it with a cloth.

Alternatively, you can make a paste from the vinegar by adding some flour and salt to it (e.g., 1 part vinegar, 1 part flour, and 1 part salt). Rub the bracelet clean with the paste and rinse thoroughly with water.

Cleaning Copper with Worcestershire Sauce
You can also use Worcestershire sauce to clean your copper bracelet. Rub some sauce onto the piece and clean it with a cloth or a soft toothbrush.
Then wash the sauce off with water and dry the bracelet with a soft cloth.