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.