When a device is first added to the Garmin Express™ software, an option to add a nickname for the device will be given. If this initial step is skipped, or a new name is desired after adding the device, the nickname can be changed later. It is not necessary to have the device connected to Garmin Express to change its nickname.
Select your operating system below to reveal instructions.
Open Garmin Express. Use one of the following three methods (examples pictured from Windows 10):
Double-click the Express icon in the system tray.
Search for Garmin Express in the Start menu.
Double-click the desktop shortcut (if present).
Right-click the desired device.
Select Edit Nickname.
Type the desired name.
Select Save.
The new nickname will now display in Garmin Express.
Launch Garmin Express from the Applications folder in Finder. The location of the 
Finder icon may vary depending on the position of the Dock.
Select the desired device (Garmin DriveAssist™ 51 LMT-S pictured)
Select Tools & Content
Select the 
edit icon next to "Nickname"
Type the desired name.
Select Save.
The new nickname will now display in Garmin Express.