Help with locked computer and changing password

My power cord died and I had to order a new one. By the time I got the computer back on, it told me that my password had expired and I needed a new one. I put in my username and the old password and it does not recognize it. It says one or the other is incorrect. I know what the username and password were, as they haven't changed in 18 months. I cannot unlock the computer. I don't even need the password protection, as the laptop never leaves the house. Does anyone know how I can bypass this option or circumvent this step. I really need to get into my computer. Thank you so much,

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