Here is the process that should answer you question. Make sure the phone or tablet, whatever is being used as the Bluetooth source app, is connected to your PC via USB. Remove any USB cable from the amp before starting this procedure. 

1. Connect your device to the PC via USB, log on to the web site, activate My Gateway through the web site, log on to MyMarshall through your device. Also make sure amp is on and Bluetooth connectivity is established. Any preset is transferred to the amp via the device using Bluetooth, not from the computer directly to the amp.

2. Add / download any new preset to your Library then move those new presets into the main Library. Any preset position you overwrite will cause the previous preset to move into your Library.

3. Click SAVE. Go to your device, open MENU, select MY MARSHALL, select SYNC NOW to ensure that the Library list goes to your device. The SYNC  does an automatic update but pressing SYNC NOW will at least allow you confirm the action. Close MENU.

4. On your device, tap the icon on the lower left of the screen to change the Library to PHONE mode. Colour should now be blue, not gold / yellow.

5. Use a swipe to scroll through each new preset. What should happen is that each preset name will appear on the amp display. The EXIT/STORE indicator should also blink. This is because you are, in essence, making an edit to an existing preset. Press and hold the EXIT/STORE button until the display says Saved. Repeat this for each new preset until you have gone through your list. You could also try using the Library and tap each setting but I found it not as reliable as a swipe across the top of the home screen.

6. All preset listings should now match... amp, device and MyMarshall Gateway.

Hope this makes sense to you and helps. One other thing... take some time and explore the Forum. This question has been answered many times. Using the TAGS will save you a lot of random hunting. Let me know if this does work for you.

Regards.