This should be in supermechs, not battledawn
Not all weapons can get past epic. If you click on the weapon, whatever it is, you will see something like this:
See those colored circles next to transform range? Those tell you all the rarities the weapon is available at.
grey for common, blue for rare, purple for epic, yellow for legendary, and orange for mythical.
What is the name of your weapon? If you can’t transform it, it might not be able to reach any rarity higher than epic.
If it can, however, simply upgrade it the same way you would boost an item and it will give you the option to transform