How to reset MacBook Air password when WiFi changed and recovery mode isn’t working?

First try this after you booted from macOS recovery

1. From the Utilities menu in the menu bar, choose Terminal.
2. In the Terminal window, type resetpassword, then press Return to open the Reset Password assistant.
3. If asked to select an admin user you know the password for, click “Forgot all passwords?”.
4. At the Reset Password window, click Deactivate Mac, then click Deactivate to confirm.
5. If you see an Activation Lock window, enter your Apple Account information, then click Next.
6. At the Reset Password window, enter your new password information, then click Next. (If this window shows multiple user accounts, click the Set Password button next to each account name, then enter the new password information for each account.)
7. When the password reset is completed, click Exit.
8. Choose Apple menu  > Restart, then log in with your new password.

If this is not working try this

1. Shut down your Mac, then start up from macOS recovery as described previously.
2. When asked to select an admin user you know the password for, choose Recovery Assistant > Erase Mac from the menu bar.
3. From the Erase Mac window, click Erase Mac, then click Erase Mac to confirm. 
4. If your Mac restarts to a flashing question mark, press and hold the power button for a few seconds until your Mac turns off.
5. Start up from macOS Recovery again, then reinstall macOS. Learn how to reinstall macOS. 

See this link for further reference

https://support.apple.com/102673