If you already have Meshcore installed (but not the bootloader), download your current settings:

• Go to https://config.meshcore.io/
• Connect your device to your computer with a USB cable.
• Click on Connect
• Click on Export
• Click on Disconnect
• Locate the .json file (probably in your Downloads folder).

Download the bootloader:

• Go to https://meshcore.io/flasher
• Select your device
• Select Repeater (or Room Server)
• Click on Download
• Select the *bootloader-x.x.x-OTAFIXx.x.uf2 file
• Locate the .uf2 file (probably in your Download folder).

Copy the bootloader over to your device:

• Connect your device to your computer with a USB cable.
• Double click on the reset button.
• Open the drive that just mounted on your computer
• Copy the .uf2 file to the drive.
• The drive may close on its own right away. This is normal.

Flash the Meshcore Firmware:

• Go to https://meshcore.io/flasher
• Select your device
• Select Repeater (or Room Server)
• Select Enter DFU Mode
• Select Erase Flash
• Select Flash

Re-upload your settings:

• Select Configure via USB from the previous page, or navigate to https://config.meshcore.io/
• Select Connect
• Select Import
• Select your .json file
• Scroll down to the Access section and retype your New Admin password (it doesn't remember it)
• Scroll down and select Save settings
• Select Disconnect

Setup the Over the Air (OTA) update app (first time only):

• Download the “nRF Device Firmware Update” app by “Nordic Semiconductor ASA” (Android, Apple)
• Open the DFU app
• In the settings, check Packet receipt notification

Perform an OTA update:

• Using your phone's browser, navigate to https://meshcore.io/flasher
  • Select your device
  • Select Repeater (or Room Server)
  • Select Download
  • Select the firmware zip file (not the bootloader file)
• Using the Meshcore app, login to your repeater (or room server) using the admin password.
  • For Room Servers: go to Remote Management
  • Go to the Command Line
  • Type start ota, you should get a message that says: OK - mac: xx:xx:xx:xx:xx:xx
• Open the DFU app
  • Under File click Select to chose the firmware .zip file
  • Under Device click Select and chose the device that has DFU in the name.
  • Under Progress click Start

See Scott Baker's article for more information.