How to reduce or shrink a partition in windows 10?

Unfortunately, there's no built-in Windows 10 tool to directly shrink a partition Windows 10 in the way some might imagine, like a simple "shrink" button.  Windows 10's Disk Management utility can shrink a partition, but it does so by moving the partition boundary.  Any free space created at the end of a partition is not immediately usable.

The key is to understand that shrinking a partition involves relocating the partition boundary to create free space.  This free space is not immediately usable; it's a gap.  To be useful, you need to create a new partition out of it (or format and use it as unallocated space).

How to shrink partition in Windows 10:

1. Backup your data: This is crucial.  Any data on the partition you're shrinking could be lost if you make a mistake.

2. Open Disk Management:
Search for "disk management" in the Windows search bar.
Click on "Create and format hard disk partitions" (the Disk Management utility).

3. Identify the partition you want to shrink: Look for the partition you want to reduce in size.  Note its drive letter and size.

4. Right-click on the partition and select "Shrink Volume":

5. Specify the amount of space to shrink:  Disk Management will show the amount of space you can shrink.  Be cautious here.  Do not shrink more than necessary to avoid losing data.  If you're not sure, shrink a smaller amount.

6. Enter the amount you want to shrink (in MB):  Enter the desired amount of space to shrink and click "Shrink".

7. The system will shrink the partition:  This process takes some time.  Do not interrupt it.

8. You will see unallocated space at the end of the partition:

9.  Crucial Step: Don't format the unallocated space yet.  This is where many people make mistakes.  Formatting the unallocated space will erase any data.

10. (Optional but recommended) Create a new partition or extend an existing one:
    * Right-click on the unallocated space and select "New Simple Volume."
    * Follow the wizard to create a new partition (or optionally extend an existing one if you have another partition you want to grow).  If you just need free space, you can leave it as unallocated for now.