You can take it, but it won't gain you a certification - the delta exam is only of value for VCP6-DCV holders (before their VCP6-DCV expires)
There are various paths to VCP6.5-DCV, check here: VMware Certified Professional 6.5 - Data Center Virtualization
If your VCP5-DCV has expired it's Path 4, if it has not expired it's Path 2.
Technically, you could also pass 2V0-621D and 2V0-622D - that would give you VCP6-DCV and VCP6.5-DCV, but given that Path 2 above only required passing a single exam (2V0-622), that's probably the sensible option.
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Although I am a VMware employee I contribute to VMware Communities voluntarily (ie. not in any official capacity)
VMware Training & Certification blog
