No change log seems to be available but you can get upgraded via app world or OTA with the links below.
Instant Messaging Clients have been updated to version 2.5.108. I use MSN and Gtalk and I looked at Yahoo Messenger so I know those have received the update, The latest update should be available via your BlackBerry App World, so you can download via BlackBerry App World. The OTA links do not currently have the newest version showing.
Open App World on your Device.
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