To level up an item you go to:
Game Home press
go to Customize then > Item menu > Item level up.
Select the item you wanna level up, then select items you don't use
for the level up. You will loose the selected equipment and have enough Happy Stars on your account. Repeat level up till your at max. level 50
Offering Star Items for level up is much quicker , but you might want to remove some buffs of the items cause they will go while level up.
To remove Buffs that are special abilities for weapons and such.
Game Home
Customize Item Menu > Item Remodeling >Remove Buffs.
You can later use them to strenge you're equipment by Adding those buffs on you're Gear in an empty slot or replace a existing one in the Add Buff Section.