Well, there is no upgrade path, so the quickest way is to re-image.
Download the RC build of the HPC software from connect.microsoft.com. If you have Infiniband cards, download the latest WinIB-ND drivers (1.4.0.2577) from http://www.mellanox.com. 
 
1. Re-image the head node and install the latest HPC pack.
2. Unzip the WinIB package on the head node, e.g. to c:\ib.
3. Open device manager and update the drivers for the Infiniband adapter, then the openib adapter. 4. Point the wizard to c:\ib\inf to select the appropriate IB driver. Add any other drivers as required.
5. Build a new o/s image (or re-use a previously built one) 
6. Click on "manage drivers" in the to-do list. Point the wizard to c:\ib\inf to add the drivers to the image
7. Create a new node template with the image you built
8. Reboot the compute nodes and wait for re-deployment to complete
9. Use clusrun to copy c:\ib from the head node to the compute nodes
10. Use clusrun to run c:\ib\inf\ndinstall -i on all nodes and thus install the new ND provider.
 
If you are using OpenSM, you'll find a new version of it in c:\ib\tools.
You may also have to re-boot the Infiniband switch after the driver update. I haven't figured out why yet, but IPoIB worked without problems, MPI over ND did not just after the driver update. Rebooting the switch seemed to fix this.