Change value inside PSObject

I import a csv file successfully and want to change one value in the PSObject array $ds.

$ds = import-csv test.csv -UseCulture
#test.csv content
#A;B
#11;22
#33;44
$ds
$ds.B[1] = 9
$ds
This does not work and nothing else either.
This should be very easy I guess.
But how ??
Ari