How do I manually change the Partition Type GUID

Since you already have Windows installed...

Windows DiskPart can easily do that. Select the appropriate disk and partition, then use the set id subcommand.

set id=7C3457EF-0000-11AA-AA11-00306543ECAC

Reference and documentation: https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/set-id

Wikipedia has a description of the structure of the GPT partition table. In short, the protective legacy MBR is at block #0, the GPT partition table header at block #1, and the actual partition entries (128 bytes or more each) are located in block #2 and subsequent blocks.

At Sourceforge, there is a command-line GPT partitioning utility gdisk available not just as source code, but also as ready-for-use compiled executables for both Mac and Windows, which can change the partition type GUID to any value you want.

But if you plan to edit the partition table manually, there are two things you should know:

• There are two CRC32 checksums you must also update after editing the partition table, or else the edited partition table won't be valid, and the system will automatically use the backup partition table instead (see below).

• There is also a backup GPT partition table at the end of the disk, which needs to be modified too... with its own two CRC32 checksums.