To download older versions of QuickBooks:

Go to the QuickBooks Downloads website.
Select the country from which you are using QuickBooks.
Select the QuickBooks Desktop version you want to install.
Select the year for the version of QuickBooks you want to download and use for your company.
Click the Search button.
On the next page, click the Download button.
Select the location where you want to save the downloaded file.
Click Save.

Once the file has been downloaded, double-click it to start the installation process.

Important: QuickBooks only supports older versions of QuickBooks for a limited time. After a certain period of time, QuickBooks will no longer provide support or updates for older versions. You can find more information about the QuickBooks Desktop service discontinuation policy on the QuickBooks website.

Here is a small summary of the steps above:

Go to the QuickBooks Downloads website.
Select your country and QuickBooks Desktop version.
Select the year for the version you want to download.
Click Search and then Download.
Save the downloaded file and double-click it to install.

Please note that QuickBooks no longer supports older versions for an unlimited amount of time.