While you cannot extend unallocated space (since your operating system doesn’t recognize it as usable space, you can extend volumes with extended space to give them more storage.
Right-click the Start button and click Disk Management.
Right-click the volume you’d like to extend, then click Extend Volume.
In the new window, click Next.
Make sure that the disk with the unallocated space is in the Selected column. Press Next.
Click Finish.