I would like to download and install version 21.1 of Dreamweaver. Is it better to delete the existing program beforehand, afterwards, or does it not matter?
Personally, since the introduction of the Creative Cloud delivery system for Adobe programs, I don't delete the older version until I know for sure the new one will work for what I need. It saved me a lot of grief when Adobe switched from "all previous versions are available" to "the two most current are the only ones you can get" policy. That would have been a nightmare when DW 2018 was released.
Typically, each new full version (2020 and 2021 for example) will install fine on the same machine, they just won't run concurrently, so I leave the old one on until I decide the new one is to snuff. Right now, I have DW CC2015 and 2021. I use 2015 for production and 2021 is there for evaluation purposes.
Dot releases automatically overwrite their full version, so when they make a 2021.3, it will overwrite any other version of 2021 on that machine. Just update from within the CC Desktop App and everything should be fine if you're going from 2021.0 to 2021.1, just make sure to restart your system before opening the new program.