Using my renown to upgrade to a new U Boat

[Hans Hessler]

In the career mode I have made 4 missions and acquired renown.
I have sailed these missions in the Type IIa Boat. I want to
upgrade to a Type VIIb Boat. I have enough renown to do this.
MY QUESTION: How do I go about this. I got to the screen that
has the missions, crew and logs on it and click U Boats. From this
screen I can view the different U Boats, conning towers etc I can
use my renown on, but from there I can't figure out how to use my
renown to upgrade to the type VIIb Boat. I don't want to sail another
mission on the Type IIa boat, can I snag some help from you seasoned
U Boat Captains on this? Thanks;

Hans