How to improve Varric's crossbow? (Dragon Age: Inquisition)

remove it from the character and improve it in skyhold or in the fortresses captured (about fortresses, I'm not sure if it needs to be removed)

kacheruuk
yeah, there are sketches for Bianca's shoulders, a handle and a sighting device, everything is crafted and put on a crossbow

Put a dragon bone on your shoulders and the varick will damage as hell.

Verstalchik
And who can improve it and how? And why remove it from the character? The sight handle and so on is understandable, but how can we improve it further?

mercury32244
itself, at the improvement machine, you only need to switch to Varric, otherwise you cannot select the crossbow