You can try shrinking your partition down using the windows disk management tool and then deleting the extra space so that it becomes unallocated.
Note: that if you have two partitions you'll probably only be able to shrink the one furthest to the right in the disk management tool.
So, if you only have 1 partition (C:\\) then you're all good.
If you have C:\\ and D:\\ for example, you'll probably only be able to shrink D:\\ down and then delete all the space after D:\\ so that it becomes unallocated disk space.