Cheat Method
     To  have  the needed amount of renown to get the upgrades to your
sub or to change your U-boat model use the following procedure:

You have to be in the base after a mission for this cheat to work. 

     Note:  This  procedure  involves  editing  a  game file. Create a
backup copy of the file before proceeding.

Enter the "\sh3\data\cfg\careers\" folder. 
     Select the name of the folder that is the same as the career that
you  are  using.  Use  a  text  editor to edit the file Patrols_0.cfg.
Scroll  down  to  your  last  mission details and change the amount of
DeltaRenown that you got paid.

example (DeltaRenown=498.750000 Change to DeltaRenown=6500.750000) 

Save the file and start the game. You can now buy what you like 
in the game.