Old dreamweaver

If the older version you're talking about was an older CC version...

1. Open the CC Desktop App
2. Click the Apps Tab
3. Click the small arrow by the DW button

4. Choose Other Versions

5. Click and install the older version

If it was older still, and part of CS software, you'd need to reinstall from your disks.

In the future, when installing new versions via the CC Desktop App, make sure to click the Advanced Options link at the bottom of the installer dialogue so you can turn off the "Remove Older Versions" checkbox.

You can keep older versions installed without losing them, well, the full version updates anyway. A dot release will overwrite the older version, so 2107.0 will be overwritten by 2017.1 for example, but 2018 won't overwrite 2017 and both versions will continue to function independently.

As of now, you can have all of the major CC releases installed on the same machines at the same time, the just won't run concurrently.