How to upgrade installed pack from python2 to python3

Recently, I am going to upgrade Stackstorm to 3.4. From release log,pack with python2 action runner is no longer support? I have 5 packs with python2, and have not certified with python3. I need to upgrade pack to python3 before Stackstorm upgrade.

Could you provide document or instructions for pack upgrade?

If you just have actions and not sensors, then on earlier StackStorm versions 3.3 etc, then you could install a pack and tell it to use python3. (As long as you had installed python3, and configured that option).
There are some instructions on running with python3 on earlier STackStorms here: Packs — StackStorm 3.3.0 documentation