In Sourcetree, "downloading" an old commit typically refers to one of three things: viewing a specific file's previous version, rolling your entire workspace back to an older state, or exporting changes between versions. Because Git is a version control system, you don't "download" individual commits from a server like a file from a website; instead, you or reset to them to update your local files.
To make your local folder reflect the state of the project at a specific point in time: sourcetree download old commit
If you only need a copy of one specific file as it existed in the past: Open the tab (CTRL+2 on Windows, Command+1 on Mac). Select the commit that contains the version you want. In Sourcetree, "downloading" an old commit typically refers
Alternatively, for more advanced needs, you can use a Custom Action to run a git archive command that bundles these files into a ZIP. Select the commit that contains the version you want
Double-click the desired commit in the History log. Sourcetree will warn you about a "detached HEAD" state. This is fine for viewing, but if you want to make changes, right-click the commit and select New Branch instead to give that point in time a name.