how do I download an update of unity without need to install the whole engine again?
There are no “updates” for the Unity editor. There are simply too many things which potentially changes between different version that any kind of “differential upgrade” would be just crazy. Almost all software that releases frequent “updates” actually ship the whole software.
Apart from that while Unity tries hard to be able to upgrade a project to a newer Unity version, the reverse is almost never supported. You usually don’t change the Unity version in a serious project once started. Hence we have the Unity Hub to manage different projects with their different Unity versions.
What’s the actual concern you have? Do you fear the download size? Or the harddisk space?