You must have FRIENDLY_NAME enabled in zigbee2mqtt configuration.yaml. If you followed my tutorial install zigbee2mqtt openhabian Then you can use the following.
Go to the Zigbee2mqtt folder.
cd /opt/zigbee2mqtt/data
Edit configuration.yaml. file
sudo nano configuration.yaml.

Here you can edit the friendly names of your zigbee2mqtt devices. Ctrl +X to exit and save.
Thing example Tradfri outlet
Thing topic WalloutletCoffeeDevice "Koffie" @ "DiningRoom" {
Channels:
Type switch : state "state" [ stateTopic = "zigbee2mqtt/WalloutletCoffeeDevice/state", commandTopic = "zigbee2mqtt/WalloutletCoffeeDevice/set/state", on="ON", off="OFF" ]
Type string : update_available "update_available" [ stateTopic="zigbee2mqtt/WalloutletCoffeeDevice/update_available"]
Type number : linkquality "linkquality" [ stateTopic="zigbee2mqtt/WalloutletCoffeeDevice/linkquality" ]
Type datetime : last_seen "last_seen" [ stateTopic="zigbee2mqtt/WalloutletCoffeeDevice/last_seen" ]
Items example Tradfri outlet
Thing topic WalloutletCoffeeDevice "Koffie" @ "DiningRoom" {
Channels:
Type switch : state "state" [ stateTopic = "zigbee2mqtt/WalloutletCoffeeDevice/state", commandTopic = "zigbee2mqtt/WalloutletCoffeeDevice/set/state", on="ON", off="OFF" ]
Type string : update_available "update_available" [ stateTopic="zigbee2mqtt/WalloutletCoffeeDevice/update_available"]
Type number : linkquality "linkquality" [ stateTopic="zigbee2mqtt/WalloutletCoffeeDevice/linkquality" ]
Type datetime : last_seen "last_seen" [ stateTopic="zigbee2mqtt/WalloutletCoffeeDevice/last_seen" ]