how do i reset password on sign in screen on macbook air

OS X: Changing or resetting an account password - Apple ...

After you login go to System Preferences > Users & Groups and if you are the user in question, select reset password and you will be able to do so.

If you are unable to login due to a forgotten password, restart the MBA and hold the Command and R keys - then from the Utilities drop down choose Terminal. When the Terminal window opens, type EXACTLY:

resetpassword

and press Enter. When the Reset Password window opens, highlight the internal storage and the user you want a new password for. Then type the new password twice, Save, and accept the next dialog that opens. Then, bottom right of the Reset Password Window agree to update the user's home directory permissions.

Go to the apple left side of the menu bar, Restart and you are now using a new password.