i want to download older version of lightroom

And in case you’re wondering why the Creative Cloud app doesn’t show more versions, Adobe has a policy where they only provide the last two major versions of a Creative Cloud app for installation. Lightroom Classic is currently on version 12, so the Creative Cloud installer provides version 12 and 11, and nothing earlier. To get something earlier you have to go to the website posted by dj_paige.

In addition to the post of @dj_paige it's recommend to save the download files on an USB stick, hard drive or CD/DVD, but it isn't sure how long these links are available. 

https://prodesigntools.com/adobe-cc-2019-direct-download-links.html

https://prodesigntools.com/creative-cloud-2020-direct-download-links.html

https://prodesigntools.com/creative-cloud-2021-direct-download-links.html