Dump valve in the old Esprit--how to open

[lotus7racer]

The 1984 Carbureted Esprit has a dump valve. I like to replace the spring with a stiffer rate. I can't seems to open and reach the spring. There is no screw or bolts or thread to unscrew to open the internals. It looks that this BOV is pressed.
Anyone out here knows how to open this thing?

Thanks