In order to change your Windows VPS primary IP, as a prerequisite you would need to make sure you have ordered and added at least one Extra IP for your Windows VPS.
The primary IP is the IP being used for all outgoing connections for your Windows VPS. In some scenario it could be useful to change the primary IP.
Here is how to proceed.
1) Login to the Windows VPS control panel.
2) Click on "HyperV".
3) Click on your Windows VPS name.
4) Once, on your Windows VPS management panel, you may click on "Network"
5) Select the Extra IPs you would like and click on "Set as Primary"
6) Finally, Click on "Restore External IPs"
After a few seconds, you extra IPs will be set as primary. Your previous IP would keep working for you to login to your VPS and all server configuration should also remain. The Windows VPS would however use the new IP for all session related outbound connections.