Welcome to the forums/glad you became a member,
There's been far greater improvements, especially in the area of security & anti-spam (another key part) than simply upgrading the anti-virus alone will bring, but if you wish to do it out of cycle take a look at
ClamAV - Updating Version - Zimbra :: Wiki
At the minimum I'd upgrade to ZCS 5.0.26 - btw 6.0.6+ uses ClamAV 0.96 (current in that line is 6.0.10), and 7.0.0 uses 0.97