Can members modify their account details?
Can Users Update their Own Profile?
[s2Member-Profile /] Shortcode makes it easy for users to edit/update their profile upon logging into your site. See: WordPress Dashboard → s2Member → General Options → Profile Modifications for additional details and other options. Users can update their name, email address, password, and any custom fields that you've configured at Membership Levels that apply to a given user.
Can Administrators Update User Profiles?
Absolutely. Users can be edited/updated in various ways from your WordPress Dashboard at any time you like. See: WordPress Dashboard → Users. Find (or search for) a specific user and click [Edit] to access their WordPress profile, along with all of the editable fields provided by s2Member.
See also: Customizing Your Login Welcome Page