Once you have a nanoweapon that fits a hero, how do you equip it to the hero?

To equip the nanoweapons the hero needs to have reached level 211. And they are only for 5 star heroes.

Go to the hero screen and select the hero. There iwll be a square symbol on the right of the hero image area. If it has a + then you can add a weapon. Tap this and select the weapon you want to use.

You can add any weapon to any applicable hero but using the specific weapon for each hero will unlock the extra benefits.