I would use a dedicated partition tool. MiniTool Partition Wizard Free Edition http://www.minitool.com/partition-manager/partition-wizard-home.html worked well for me several times. Meanwhile I use it more often than my paid partition tools that I still have.
But I am afraid that it will not work that easy: Here is a picture of the many partitions on a Android-Windows device
I do not see how the space alloted to Android can easily be added to the Windows partition.
I did it the hard way and removed Android totally. Then of course you really get a bigger Windows.