Open the Start menu, right click 'Computer' and then left click 'Manage', give permission if required.
In the window there should be a page in the middle with 'Storage' among two other options, double click storage and then 'Disk Management.
Wait a few seconds for all your drives and partitions to load. Right click the partition you wish to extend and continue from there, or if you want to create a new partition you should right click the unallocated space and select create partition.