There was an option for this in about:flags, but that interface is unusably broken since IIRC build 10130 and still does not work in RTM as of July 24.

However, this registry setting still works to enable EdgeHTML in IE:

[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main]
"DisableRandomFlighting"=dword:00000001
"EnableLegacyEdgeSwitching"=dword:00000001

Edit: In Windows 1511 (build 10586), this does no longer work.