NT: Changing Partition size

I am using Windows NT.
I have 3 partitions, plus about 2 Gb more of un-partitioned free space.
I could create a 4th partition with this free space.
But what I would like to do is increase one of the partitions in size by that amount with-out losing the data.
The NT disk manager will not let me change the size, or maybe I am doing something wrong.
Can anyone help me?