There are no instructions for W8.1. On all systems (Linux, windows, OSX, android) you do it in exactly the same way.
1. Download firmware (2 files) for your router - v1 or v2 - it depends on version of your MR3040.
2. Install "factory-image" as firmware update though router management web site
3. Reboot router, it will have OpenWRT now.
4. Install "sys-upgrade" through OpenWRT sys-upgrade option.
5. Reboot router
6. Configure router as you like.