daisymcfadden , Open Settings on your phone or tablet, scroll to About and tap on it. On the next screen, tap on Device Name, type the desired name for your device in the text field and select OK. If you have an HTC device, go to Settings> About> Phone identity and tap on Phone Name. Type your custom device name and done. Your manual is posted below for more info. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
https://support.t-mobile.com/docs/DOC-7362