Windows Disk Management cannot extend a disk if it has a lower offset than the selected partition, i.e. it is on the left of selected disk. So what to do, use the disk managers discussed here:
- How can I extend a volume where the unallocated space is located to the LEFT of the Primary Partition in Disk Management
- Windows - Adding Unallocated Drive Space "From the left"
Basically first the partition will be moved left then extended. See the steps in the second link.