Which password Bruno?
Password for your email account? For this you will have to contact your email provider. Most have a password reset link on their webmail signin.
Password for your eM Client Pro Licence? This you can reset on the License Manager page. The credentials will be resent to the registered email address.
Or password that you set for accessing the em Client application? This cannot be reset. You will need to delete the eM Client settings. To do this, close eM Client, then delete C:\Users\yourusername\AppData\Roaming\eM Client\settings.dat. Delete any other settings files as well. Then restart eM Client.