Anything that relates to your campaign can be posted as an update. Be it signing an agreement with the supplier, sharing a short video of the manufacturing process of the product, or posting images of your rewards ready for bulk shipping. Anything to keep your backers on the edge and ready to receive their rewards.
Some campaigners may choose to update everyday, some may choose to update once every few months. Find out what works for you. In general, Posting updates on your milestones would suffice.