You can make a collar narrower but not wider.
On a jacket, this involves undoing some of the lining, resewing the collar edge, and re-doing the lining. If the original collar is hand tailored and beautiful, leave it be or take it to an old-school tailor. Hand tailored collars (and lapels) have a roll and body that can't be replicated by modern tailoring techniques (mostly, they involve sewing the layers of the collar flat and thus the finished product is flat).
On a shirt, this involves a lot of work for very little difference. There's lots of stitching to be undone, all of it on areas with minimal seam allowance to begin with, and it's generally a fiddly process. If you love the shirt, it can be done, but don't expect your tailor to be happy about it.