How do I install a version of lightroom that works with Windows 7

the last version of lr developed to be compatible with win 7 was lr cc 2019, All the New Adobe CC 2019 Direct Download Links, Now Available! | ProDesignTools

you would need an active subscription to use this beyond the trial period and you need to follow the directions to download,

(esp step 1)

save the install file. it won't remain online.

thank jim @ prodesigntools.com

if you already have a valid license for lr 6, follow @John Waller's suggestion.

Are you sure that your computer no longer works with Windows 10? What are your system specs? It's possible there is a deeper issue with the computer going on that re-installing an old, unsupported version of Windows will likely not resolve.

need to download the old version I used to use on Windows 7

Assume you mean Lightroom 6 and that you have a valid serial number for Lightroom 6.

The Lightroom 6 installer can be downloaded from:

https://prodesigntools.com/adobe-lightroom-6-cc-direct-download-links.html

If not, do you have a Cloud subscription or what version of non-Cloud Lightroom do you need?

Adobe only provides programs for current operating systems

I do not know how far back this process will go, but try this link
-NOTE that you will have to search the site for your specific download
Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-the prodesign links go directly to Adobe's file server, so this process will
--only work until Adobe completely removes the old version(s) from the server
.
IF/When you do have a download, be sure to copy it to AT LEAST one external device
-since Adobe does sometimes remove old versions from the server
-a usb flash drive or external hard drive, or write to a DVD
-plus a text file containing your serial number if you have a serial number program