Your question is a bit too vague to be sure, but I think you're asking if you can take a 6.5 database and point JIRA 6.4 at it and have it work.
The answer is "no". JIRA does not contain any code that enables upgraded data to be downgraded.
I'm not sure if completely understood your question, but since you mentioned the 6.5 version and we don't have such version available in JIRA Server yet, I believe you are mentioning a backup from Cloud. Am I correct?
If so, you can start your JIRA from scratch (JIRA 6.4.5) and import the Cloud backup (XML file generated in your Cloud instance) directly to this version. Here you can see step by step how to accomplish this.
Hope it helps!
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