The standard recommendation is Partition Wizard. I think there are paid and free versions and I'm not sure that the free version can do what you need. There was another thread here recently discussing that. Go to their web site and take a look at the documentation or download the free version and see if it's possible.
Paragon Software also makes partition tools---some of them free as I recall.
You should be able to do it with Windows own tools if you can get the data off of one of the partitions temporarily. Then delete that partition. Then expand the remaining one to take up the space left behind.