[Krupp]

After returning to your "office" and picked the U-Boat table you should see a row of icons. Conning tower, torpedo, guns, emblems etc. Click one of them you wish to change and you see a vertical list on the left side that shows you what's available in that category. Just double click on the name there and the game will ask you to confirm it. If you got the renown of course.

Hope this helps.