Windows Disk Management
When upgrading Windows 7/8 to Windows 10 or having a new hard drive on your PC, many users will turn to the Windows 10 built-in Disk Management Tool to manage their hard drive and partitions.
Disk Management is a system tool in Windows 10, 8, 7, XP that helps you to view disk information and perform several disk management operations, including initializing a disk, creating volume, formating volume, shrinking a volume, extending a volume,
Disk Management Extend Volume grayed out
However, Disk Management has some drawbacks in disk management. It is not suitable for every circumstance. For example, the "Extend Volume" enables you to increase a partition space. but it can be only applied to the situation where there is an unallocated space adjacent to the right side of the drive you want to extend, otherwise, the Extend Volume would be grayed out.
Or you can delete an adjacent partition to have unallocated space, and extend a volume with it. In this way, you will lose all the data on the partition. It is not recommended.

Alternative Freeware to Disk Management in Windows 10
Luckily, a powerful and free partition manager for Windows 10-AOMEI Partition Assistant Standard, which can help you to manage your hard drive and partition in Windows
10/8.1/8/7/Vista/XP. It can help you to resize partition, split partition, create partition, delete partition, format partition.
And this flexible software meets different requirements in disk management. Even if there is no adjacent unallocated space, it helps you to shrink a partition to generate some unallocated space that will be merged into the partition you want to extend. Besides, It also provides some advanced features, such as copy partition, convert data disk between MBR and GPT partition styles, create WinPE bootable media, Windows To Go creator for Windows 8/10.
Next, you can free download the freeware and see how to manage partition in Windows 10.
How to extend volume in free while Windows 10?
To extend volume while the Extend Volume of Disk Management grayed out, you can get help from AOMEI Partition Assistant Standard. With its help, you can make it
easy. Take the system extending as an example.
Step 1. Install and open this free partition manager. The main interface is shown in the following picture. Right-click the D partition, and choose "Resize/Move Partition".

Step 2. Drag the border of the D drive leftward to shrink the space of it.

Step 3. Now, right-click the system partition, and choose "Merge Partition".

Step 4. Tick the unallocated space and the C partition, and click "OK".

Step 5. In the main interface, choose "Apply" to save the change.

Final words
AOMEI Partition Assistant Standard is a good free partition manager in Windows 10, 8, 7. With its help, partition Windows 10 even partition Windows 10 USB drive would be a simple thing. Change drive
letters, hide partition, check partition, etc, you can do all the things for Windows
10 in a few steps.