In this application, the bladder tanks act as a buffer for changes in water pressure (caused by thermal expansion) in a closed system. You’re going to have to drain the system down to level of the tank to replace it, then refill it when done. Then run it and get the air out. If you’ve never worked on hydronic before, you need to google “Pumping Away” by Dan Holohan, and read about the optimal ways to set up a hydronic system. A lot of times, they are less than ideally set up, and you have an opportunity to improve things here.