Sumo Logic will save older versions of the collector for 1 year based on the time it was released.
To download a specific version you will need to use wget and add the version after the URL.
For example to get the latest version of the collector.
To download the collector version "19.216-33".
A list of previous versions can be found from the UI by going to Manage Data -> Collection -> Search for a Collector -> Edit -> Version.
You can also find previous versions from our collector release notes page: