To start with your gear star up. You must first obtain a King's Equipment and Evolve the tier until it reaches to 12 or the symbol fist with two line bars under it. 120 Minimum Level and the Supreme Symbol is unlocked. You will then see the Gear Star Up button that lets you evolve your King's gear.
You will need both 3x 6* Fusion Stone and 3x 6* Refined Stone to transform your 5 star gear into 6 star.
Note that only your tier level will reset if you wish to continue with your gear star up and you might lose a little bit of power for a tier 1 6 star gear and the supreme symbol will remain upon starring up your gear.
6 Star Gears are difficult to evolve so make you sure to prepare and save resources if you plan to evolve your gears.
Good Luck!