Press 'S', click the 'Action' icon, then the 'Upgrade' icon to open the Strengthen Window.

Place the equipment you want to upgrade in the window.

To know the jewel needed to upgrade the equipment, click 'Instruction' button. The items needed for upgrading a weapon, armor, helmet, glove, shoes, ring, necklace and earring are listed there.

You can use 1 up to 100 gems on one item but if you repeat the upgrading process, the old effect will be removed. The effects do not stack so it??? better to wait until you have many gems.

Not all equipments can be upgraded. You can check if the item can be upgraded by pointing your mouse over the item. 'Strengthening available' should be indicated in the item details.