If you have forgotten the admin Password, you should be able to reset it using Forgot Password link available in the login page. But if it does not work, for example sending email is disable in your server, your email is mis configured, you may not receive email to reset password. In that case, you can reset password directly from database.
Reseting Password from Database
The name of the users table is sys_users
Open the sys_users table with any mysql tool for example phpMyAdmin and edit the password field for specific user with the following contents-
By doing this, your password will be set to- 123456
Change your password immediately after login.