Solved it myself 
I first had to upgrade to Duet Web Control 3.4.0-b2, after that, I could install the 3.4b2
Best posts made by MarcFischer
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RE: Can't upgrade from 3.3 to 3.4b2posted in Firmware installation
Solved it myself I first had to upgrade to Duet Web Control 3.4.0-b2, after that, I could install the 3.4b2