Assuming you have obtained a clean .cab file (and you are only doing this for nostalgic UI exploration, not real use), here is how to install it.

Users still using HTC Windows Mobile 6.1 hardware are encouraged to upgrade to a device running Android 6.0 or newer or iOS 12.0 or newer to continue using WhatsApp. Current Supported Versions

WhatsApp never released a version for the legacy Windows Mobile (6.1/6.5) platform. Their official support for Microsoft mobile operating systems began with Windows Phone 7.5 (Mango) WhatsApp.com Key Facts About Compatibility Unsupported OS