Replacing the back panel of a Vivo V21 can be done by following these general steps:

1. **Power off the device**: Make sure the phone is turned off before starting the replacement process.

2. **Remove the SIM tray**: Use the SIM eject tool provided in the Vivo V21 packaging to eject the SIM tray.

3. **Heat the back panel**: Use a heat gun or a hairdryer to gently heat the back panel of the phone. This softens the adhesive holding the panel in place.

4. **Pry open the back panel**: Once the back panel is heated, use a plastic opening tool or a guitar pick to carefully pry open the edges of the back panel. Work your way around the edges gradually until the panel loosens completely.

5. **Disconnect any cables**: Once the back panel is removed, be cautious of any cables or connectors attached to it, such as the fingerprint sensor cable or camera cables. Disconnect them carefully.

6. **Replace the back panel**: After disconnecting everything, place the new back panel in position and align it properly.

7. **Reconnect cables**: Once the new back panel is in place, reconnect any cables or connectors that were disconnected earlier.

8. **Reinsert the SIM tray**: Put the SIM tray back into its slot.

9. **Press down to secure**: Apply gentle pressure around the edges of the phone to ensure the new back panel is securely in place.

10. **Power on the device**: Turn on the phone to ensure everything is functioning correctly.

If you're not comfortable performing these steps yourself, it's advisable to seek assistance from a professional technician to avoid damaging your device. Additionally, make sure you have the correct replacement back panel for your Vivo V21 model.