Can the mount on a cannon lens be changed to fit a nikon d90 camera?

Can the mount on a cannon lens be changed to fit a nikon d90 camera?

It may not suit.

Short answer: No.

Canon lenses have a bigger diameter than Nikon ones and thus can't be made to fit. In the opposite direction adapters are available to mount Nikon lenses on Canon cameras, but these have serious drawbacks. There's very little reason ever to use such an adapter, most often selling the lens that one obtained and buying one of the appropriate brand is a better solution.

Yes only if the lens has in interchangeable mount like those from Tamron and Sigma. Those from Canon can't be modified.