How to edit every node of a content type on cron run?
You would need to implement various hooks. 

hook_cron: It will get called on every cron run. Here you could load every node in the system using node_load(), edit whatever properties of the node you need to edit, and then save each edited node using node_save().

However, note that both loading and saving a node is an expensive process, and if you have a site with many nodes, your site would become very slow, unstable, or even unusable on every cron run (which can sometimes happen as often as every 15-30 minutes). In other words, I would be very cautious with whatever you are trying to accomplish.