Upgrading
To uprgade your equipment you will need Heavenstone and it to be level 30. When you have the required Heavenstone (weapon, armour, or accessory) do the following.
1) Go to Smithy and tap the Upgrade tab
2) Select the Gear to be upgraded
3) Check the upgrade cost and required Heavenstones
4) Confirm upgrade
Finding Heavenstones
You cannot buy Heavenstones, below is a list of several ways you can obtain them.
There is a possibility of finding 1 or 2 random Heavenstone as loot when you are farming or clearing a dungeon.
Guild Battle
Either join or create a guild and reach Class SSS so you win the Daily Guild Battle Reward which is given every 24 hours. Heavenstone is only one of the possible rewards, you can also get potions, gold Crystals and armour.
The Daily Dungeon is the best way to get Heavenstone if you are strong enough to deal with the Bosses and know the schedule.