mine had the same issue, grab some pliers and pull it out, its pressed in. be careful not to drop flakes and pieces of rusted metals inside when pulling it out. mine was so bad it was coming off in pieces and took forever. Once Removed, got some sandpaper and cleaned it up. clean up the grove in-which the fitting seats in. you want this to have zero pieces of debris left over to not cause leaks later on. Added some Permatex to the outer diameter of the new press fitting, a little bit to the groove too, grabbed a rubber mallet until seated the fitting. fairly easy job.
The problem is the fact we have Stainless Steel in direct contact with cast iron, these are very dissimilar metals and causes galvanic corrosion especially with the coolant in the mix. Hence, why its leaking after 52 years. The sealant should help you never have to touch this issue again.