how can I change a device name?

Change the name of your iPhone, iPad, or iPod - To change the name of your iPhone, iPad, or iPod touch, follow these steps: On your iOS device, go to Settings > General > About. Tap the first line, which shows the name of your device. Rename your device, then tap Done.

https://support.apple.com/HT201997

Settings, general, about, name.

Settings > General > About > Name > Name [Change]

Restart the device. That's it.