If you forgot your Windows 10 password, the easiest way to get back into your account is to reset the password for your Microsoft account. You'll need internet access, but it's the usual way to sign in, and it allows access to services like Outlook and Skype. If you can't sign in that way, you'll need to use a local account. A local account signs you in on your device offline, and doesn't link to your other devices.
Reset local account password for Windows 10, version 1803 and beyond
If you added security questions when you set up your local account for Windows 10, then you have at least version 1803 and you can answer security questions to sign back in.
After you've entered an incorrect password:
Select the Reset password link on the sign-in screen. If you use a PIN instead, see PIN sign-in issues. If you're using a work device that's on a network, you may not see an option to reset your password or PIN. In that case, contact your administrator.
Answer your security questions.
Enter a new password.
Sign in as usual with the new password.
Reset local account password for Windows 10, before version 1803
For versions of Windows 10 earlier than 1803, local account passwords can't be reset because there are no security questions. You can reset your device to choose a new password, however this option will permanently delete your data, programs, and settings. If you've backed up your files you'll be able to restore your deleted files. For more information, see Recovery options in Windows 10.
To reset your device, which will delete data, programs, and settings:
Press the Shift key while you select the Power button > Restart in the lower-right corner of the screen.
On the Choose an option screen, select Troubleshoot > Reset this PC.