If you want to enhance an item, you can go to the Armory.
All items except rings and necklace can be enhanced: weapon, glove, helm, cuirass, belt and shoes.
How to Enhance
Go to Armory and choose Enhance, drag the equipment you want to enhance to box and you'll see the success rate of enhancement, current stats, price, etc. The higher the synthesis level is, the lower the success rate will be. Price will be higher as synthesis level rises.
When all preparations are done, click Enhance to start. If enhancement succeeded, a new window will appear, showing the stats before and after the enhancement. You can choose continue enhancing or not.