The download cache can be killed from the settings menu; go to Downloads and from there, you’ll see a button toward the bottom which says “Clear Download Cache” (it used to be trickier back in the day, but now that part is pretty simple). After doing that, you can log out of Steam and go into your Steam directory. One of the files there (in the main directory) will have a .blob extension/file type. That saves your login info. You can delete it or simply rename it, and then when logging in again you’ll enter your user ID and password like it’s the first time and Steam will create a new “blob” file.