How to exchange/sell spare blueprints?

The blueprints do not go into the machine they can be exchanged by the guy that you can upgrade weapons with blueprints. He is one of the guys at the bottom of the stairs you go up to see mayor you click on him and then on the left there is a refresh symbol. Hit that and it takes you to the spot you can exchange blueprints but be aware you need at least 2 blueprints to recycle for another this is the same guy you can take items and weapons with 100% and turn them into blueprints to upgrade weapons

The guy at River City that upgrades the blueprints has another tab that allows you to trade two known blueprints for a random one.