How can I get the older version of final cut? If my computer can't update any more ?

Exactly what computer do you have, and what version of the OS? Unfortunately Apple only sells the current version, which means you have to find a way to buy the current version from the App Store, before you can download an older version for your computer.

I'm not sure which version you purchased before. If it was the legacy Final Cut Pro, version 7.0.3 or earlier, the current application is completely different and has to be bought separately. If you bough Final Cut Pro X after 2011, you should be able to get the version you need. Log into the App Store with the Apple ID that was used to buy FCP you should find it listed on your Purchased page.

Are you logged in to the App Store with the same Apple id, and clicking your account at the bottom left? Are you seeing Final Cut Pro in the list of your purchases?

You can’t buy an older version directly, but if you have bought FCP you can get an older version.

What is your mac and what version of the OS fo you have?