How can i uninstall (and reinstall) misbehaving Preview on Mac?

Previously I took screenshots with Preview (comes with Mac), then embedded those images inline in text body of gmails. Suddenly now, Gmail insisting on their being attachments only, and often the attachment downloads on the other end as a corrupt file (even tho the screenshot looks fine upon first snapping). I would like to update Preview or uninstall and reinstall it, but can see no way to do that. Also don't know whether is is a Gmail issue or a Preview issue?