You can use Equipment Specialization to improve equipment you otherwise do not need.
You can tap on the [Equipment Specialization] menu from the main menu (≡) to view the current equipment stats and its improved stats.
You can also use your equipment as materials for Equipment Specialization.
The amount of stats increased will depend on the type of equipment used as materials.
The better the Grade of the materials used, the faster the stat levels will increase.
You can also make use of the [Auto-Select] button after selecting the Grade of the equipment on the bottom of the Equipment Specialization screen to select in batches.