This screen lets you edit your account settings: last name, first name, email address, language.
If Mail notifications is checked, you will receive notifications about any event that occurs on the projects you belong to (issue added or edited, new document,...).
If it's unchecked, you will only receive notifications about the issues you're involved in (author or assignee) and about things you watch (eg. issues, forums).
To change your password, click "Change password" on "My account" page and then type your old password and your new password twice. Password length must be 8 characters or more (the admin can change the minimum length. See RedmineSettings).
If your account uses an external authentication mean (LDAP), you can't change your password in Redmine.
If you forget your password, this procedure lets you choose a new one.
On the login screen, click on Lost password. Enter your email address and submit the form. An email is then sent to you. It contains a link that allows you to change your password.
Note that this functionality can be disabled by the administrator (see RedmineSettings). Also, if your account uses an external authentication mean (LDAP), this procedure isn't available.
By registering, you can get an account without the intervention of the administrator.
In the upper right corner of the login screen, click on Register. Fill the form and submit it. An email will be sent to you. To activate your account, use the link that is contained in this email. See Register for more details.
Note that this functionality can be disabled by the administrator.