DO NOT do this unless you have specifically been advised to. This might render your phone inoperable. This is NOT a normal basic troubleshooting step. We will send you a firmware file if you need to do this, which you should save on your computer in a temporary location to load into the phone.
This applies to current Grandstream models such as the GXP2130, GXP2140, and GXP2160, not older models such as the GXP2100, GXP2124, or GXP2120.
Find the IP address and log in:
Once the phone reboots, press the up-arrow button to show the IP address. Log into the phone on any web browser by typing the IP address into the URL bar. The password and username are both "admin" by default. In some cases the password may be "admin1" so try that if admin fails.
Click on the "Maintenance" option near the top right then "Upgrade and Provisioning."
Click on the "Start" button. Click the upload icon, and select the file that was sent to you. Click the "Start" button, and the phone will now take a few minutes to upgrade, then reboot itself.