how do I turn off sync on my desktop

This is usually caused by OneDrive Desktop backup or OneDrive folder sync between both PCs using the same Microsoft account.

To stop your Desktop from syncing:

1. Select the OneDrive cloud icon in the taskbar.
2. Select Settings.
3. Go to Sync and backup and select Manage backup.
4. Turn off Desktop.

If the Desktop is already being synced through OneDrive folder selection, change that instead:

1. Open OneDrive Settings.
2. Go to the Account tab.
3. Select Choose folders.
4. Uncheck folders you do not want synced to that computer, then select OK.

If the goal is to stop all OneDrive syncing on that desktop:

1. Right-click the OneDrive icon on the taskbar.
2. Select Pause syncing and then Quit OneDrive for a temporary stop.
3. For a permanent stop on that PC, open Settings in OneDrive, go to the Account tab, and select Unlink this PC.

Important:

• If a folder is unchecked from sync, it is removed from that computer, but it remains available online.
• If a site or library is stopped from syncing, copies of the files remain on the computer unless deleted manually.
• Removing or unlinking OneDrive from the PC does not delete files from OneDrive.com.

References:

• How to cancel or stop sync in OneDrive
• Choose which OneDrive folders you want to sync on Windows or macOS
• how to stop pcs sharing the same Microsoft account from syncing data - Microsoft Q&A
• How to UNSYNC two laptops? - Microsoft Q&A
• Desktop messed up - Microsoft Q&A