Refit and retrofit are the processes by which a starship (or any other type of vessel) is overhauled and re-outfitted with newer technology.
A refit is the installation of updated equipment aboard a starship. It differs with a retrofit in that the vessel's overall structure is changed during the process. A refit requires some form of spacedock.
A retrofit is the installation of updated equipment aboard a starship. It differs from a refit in that the vessel's overall structure remains unchanged in the process - the components in question are simply swapped out. A retrofit may not require a spacedock.
A standard part of retrofits is to switch out the bridge module of a starship design. Bridge modules may appear radically different from previous versions, while the structure of the starship may remain the same.