You upgrade to a new boat by "purchasing" it with your "renown points". The flotilla you are based in also has to have historically operated something other than a VII type sub.
not only does the flotilla have to have a class of U-boat "higher" than the VII, but you have to have enough renown to "purchase" the new boat. So if you have been using your renown on a lot of upgrades or acquiring more experienced crew you wont have enough for a new boat.
to upgrade to a new boat, go to the u-boat upgrade screen and click on the U-boat symbol. On the left side it will show you the different types avalable as well as their "cost"
From the stock manual: (not sure which version you are using) These are the boats you will be assigned initially by year when you start the campaign. remember that when you start a career the information box about the flotilla tells you what boats will eventually be available. You may also transfer from flotilla to flotilla after completeing a certain number of patrols by clicking on the telephone on the captain's desk in the campaign "office".
1st Flotilla
1939: Type IIA
1940: Type IID
1941: Typpe VIIB
1942 - 1945: Type VIIC
2nd Flotilla:
1940: Type VIIB
1941: Type: IXB
1942 - 1945: Type IXC
7th Flotilla:
1939: Type IIA
1940: Type IID
1941: Type VIIB
1942: Type VIIC
1943-1945: Type IXC
10th Flotilla:
1942: Type IXB
1943 -1945: Type IXC
11th Flotilla:
1942: Type VIIC
1943: Type IXC
29th Flotilla:
1941: Type VIIB
1942: TypeVIIB
1943-1945: Type VIIC
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