It seems that the new 9.0.1 installation package doesn't install the IHS server binaries automatically. You must first rename the IHS folder name.
It's a minor bug, but has to be taken into account.
More information here.
Rename the current IHS folder before running the 9.0.1 install
over the top of the existing 9.0 server and the new binaries
will be layed down. But it will clobber the existing IHS conf
files with no warning.