There is no direct upgrade path from 6.x to 8.2. You have to go through 7.2 and 8.0.
Be prepared for issues on the way - app compatibility, python version change...
thanks for replying.
How can I upgrade it? I can't find the patch from 6.5 to 7.2 on Splunk download page already.
Apparently the unsupported versions are no longer available for download freely. You have to contact support directly to get the installation files.