Reflashing the mSD card is the same process as you did first time when you install Umbrel.
Just take it out, plug into a PC, use Balen Etcher with the latest version of Umbrel and done.
Keep in mind: reflashing mSD is not wiping the user data, is just updating the UmbrelOS. All user data is stored on SSD external drive.
If you want to wipe all user data please follow the Umbrel troubleshooting manual to do that.
WARNING: DO NOT reset user data, withouit having the seed backup and channels backup!