Open the navigator of your preference (Internet Explorer, Chrome, Safari, FireFox, Etc.), type in the address bar: thenameofyourcompany.com/webmail and press enter.
Type in your e-mail address and password assigned ; in case your navigator doesn’t recognize the security certificates, just click on continue, I understand the risks or continue anyway, according to the case
Finally, select the platform in which you desire to check your e-mails, (Horde, roud cube o Squirrel Mail), you can select any of the three options that you feel more comfortable with and easy to use, also on the main screen in the bottom left you can change your password just by clicking over the button Change Password.