Just resize the OS partition and you can then dual boot both Windows and Ubuntu Mate, you have enough space on the drive (I assume?). You should create a total of 3 partitions (swap isn't necessary now). a small UEFI partition of about 500MB, a root partition of about 15 to 20GB and finally a home partition with the size of your choice.
See the following guide about using the "something else method" in case you ever need to use that method!.
