How do I change my email address or password?

You can change your email address on the "edit your profile" page. Here you can change some other information including search preferences, address and other preferences about your daily routines, habits, hobbies etc.
You found this link in member panel menu, when got login.
After you've made your changes, click "Save" at the bottom of the page to save.

How do I edit my profile? How do I hide my profile?

To update your profile, click "Edit Profile" in Members Panel menu. Be sure to save your changes at the bottom of the screen when you're done.

Try to put your best foot forward with at least one photo and snappy, detailed essays about your interests. The photo is key since most people feel more comfortable writing to members they can see. Try to stay positive in your essays and let the real you shine.

To hide your profile, select "No" for the option "Other members can see when I'm online?" on the "Edit Profile" page  . Keep in mind, however, that members who are most active on the site keep their profile visible in all places because it ups their chances of connecting with people. And you never know, someone special may be looking for you and never get to find you if your profile is hidden.