HOW TO: Convert GPT to MBR with Diskpart

As per MS Official Guide/Docs

Converting using a command line

TIP: Back up or move all volumes on the basic GPT disk you want to convert into an MBR disk.

1. Open an elevated command prompt by right-clicking Command Prompt and then choosing Run as Administrator.

2. Type diskpart. If the disk contains no partitions or volumes, skip to step 6.

3. At the DISKPART prompt, type list disk. Note the disk number that you want to delete.

4. At the DISKPART prompt, type select disk <disknumber>

NOTE: Running the clean command will delete all partitions or volumes on the disk.

5. At the DISKPART prompt, type clean.

6. At the DISKPART prompt, type convert mbr.