Hi,
I currently use the latest version of Fedora (with LUKS encryption).
Unfortunately I need Windows for my work.
Thus I want to (clean) install Windows 10 first, then reinstall
Fedora and configure Dual Boot (as recommended online).
Can I simply use the Windows USB Installer-Tool and erase my
LUKS-encrypted partitions during the graphical Windows setup? Or
do I have to do something else to delete my old Fedora partitions?
Thanks for your help 
It depends.
Do you have data you need to keep and restore or can you do a totally clean install.
If you can do the totally clean install then using the Windows installer and completely erasing the existing partitions seems simplest. That will allow windows to do its thing with creating the recovery partitions, etc.
You then resize the windows NTFS partition as described for the dual boot install and reinstall Fedora.
If you have data to backup and restore it adds a couple steps but is not much harder.