Since you know how to setup dual boot configuration, the tutorial I am going to post below will not be difficult for you.
However, I would like to suggest a slight change of plan......
Instead of deleting Win 10 Home partition and keeping the Insider Preview partition, I would suggest the opposite. First delete the Insider Preview partition. Then change the Windows 10 Home to receive Insider Preview builds.
I am assuming that Win 8.1 was installed FIRST, then dual boot was setup and Insider Preview was installed afterwards. Yes ?
Logon to your Win 10 Home, and use this link......
http://www.techrepublic.com/blog/windows-and-office/remove-windows-8-from-a-dual-boot-configuration/
The tutorial is about how to remove Win 8 on a dual booting setup of Win 7 and Win 8.
The procedures are exactly the same. Just a change of "players' names ".
Skip the BCDedit part, scroll down to
Removing the partition and start from there.
In case the beginning part is not clear to you, here is how to access disk management....
Right click at Start > Run > type diskmgmt.msc in the box > OK > you are now on Disk management.
This tutorial is to.......
1. Delete the Insider Preview volume ( Win 8 volume).
2. Delete the Insider Preview partition (Win 8 partition).
3. Extend the free space back to Win 10 Home partition ( OS C: partition).
Once you are done, your computer will have only one OS which is Windows 10 Home.
How to change Windows 10 Home to Insider Preview :
Start > Settings > Update & Security > Advanced options > under Get Insider Builds, click the Get started button > follow the prompts and you will be switched to accepting Insider preview builds > set Fast ring to receive new builds.
That's it. Soon you will be getting new build, most likely build 10532.