First, determine if any Azure Resource Manager templates refer to specific versions. If so, update those templates, or keep older image versions. It's best to use version: latest.
Next, if any virtual machine scale sets refer to a specific version, you should think about whether these will be scaled later, and decide whether to keep older versions. If neither of these conditions apply, delete older images in Azure Stack Hub Marketplace before downloading newer ones. Use Marketplace management to delete them if that's how the original was downloaded. Then download the newer version.