How do I reset Sitecore admin password?

The easiest way to reset your Sitecore admin password (or password for any user) is to do it directly in the database. Sitecore uses .NET membership tables to store user data so to reset the admin password simply run following T-SQL statement in Microsoft SQL Server Management Studio.

UPDATE dbo.aspnet_Membership 
SET [Password]='qOvF8m8F2IcWMvfOBjJYHmfLABc=', [PasswordSalt]='OM5gu45RQuJ76itRvkSPFw==', 
[IsApproved] = '1', [IsLockedOut] = '0'
WHERE UserId IN (SELECT UserId FROM dbo.aspnet_Users WHERE UserName = 'sitecore\Admin')

This will reset your admin password back to b and also will make sure your account is unlocked in case you locked it out by unsuccessfully trying to login too many times.