help...my imac wont install current version of lightroom as its too old....

1. update your os to the latest you are able (and willing) to use.

2. find the latest lr version compatible with your os, Lightroom Classic system requirements (adobe.com)

3. install that version via the link given by @Ben107584 M , or if it's not available on prodesigntools.com, contact adobe support* for a link.

*

using a browser that allows popups and cookies, contact adobe support by clicking here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam.

p.p.s you can also use phone (but chat is usually quicker), https://helpx.adobe.com/contact/phone.html

This doesn't make sense unless we know what you are working with and what was the last version of Lightroom you were working on.  Best we can tell you without specs is to go to https://prodesigntools.com/tag/ddl and search through the downloads for older versions. We would be able to provide you with a more direct link with more information, but that should at least get you moving in the right direction.

What exact macOS version do you have? Itr's kind of important.