Hello Tom Moses,
Since you have already purchased the upgrade, you generally do not need to download a completely new installation file or ISO. The code for Windows 11 Pro is already present within your current Windows 11 Home installation, but the features are currently locked. The "download" you are looking for is actually just a small enablement package that switches the licensing state. Since the transaction was processed digitally, the license is attached to your hardware and Microsoft Account as a Digital Entitlement.
You should first verify that the Microsoft Store recognizes the purchase by navigating to Settings > System > Activation and selecting "Open Store" within the Upgrade your edition of Windows section. If the Store opens and shows an "Install" button, click that to trigger the reboot and upgrade sequence. However, if the Store UI is failing to trigger the download or still asks you to buy, you can force the edition switch manually using the default Windows 11 Pro generic key. This is a standard deployment step when the Store association fails to initiate automatically.
To do this, remain in Settings > System > Activation, select Change product key, and enter the generic RTM key: VK7JG-NPHTM-C97JM-9MPGT-3V66T. This specific key is designed solely to switch the edition from Home to Pro and will likely trigger an error message or a prompt to upgrade; disconnect from the internet if it errors out, then proceed. Once the machine reboots and finishes the feature update, reconnect to the internet. Your system will then communicate with the Microsoft activation servers, recognize the digital entitlement you purchased, and activate the Pro edition automatically.
I hope you've found something useful here. If it helps you get more insight into the issue, it's appreciated to accept the answer. Should you have more questions, feel free to leave a message. Have a nice day!
VP