Hey Charles, 

The since v4 the official way to do this is with the .net Membership API ala:

http://www.aaron-powell.com/umbraco-members-profiles

and

http://our.umbraco.org/wiki/how-tos/membership-providers

However, you can't change the Member's login name or the display name via the membership API, so you can fallback to the (depricated) businesslogic Member:

using umbraco.cms.businesslogic.member;

Member m = Member.GetMemberFromLoginName(username);
m.getProperty("blah").Value = "wibble";
// save and re-generate the XML:
m.Save();

TTFN

Ant

Hi, worked this out now.  Thanks for the help :)

Hi Charles,

Can you please show me your web-config, the <membership> and the <profile> part if you can, please :)

Still struggling with membership :-(...

now I am getting 

The username of a Member must be different from an emptry string
Parameter name: loginName

... When I am trying to access all the users 

and this is 

System.ArgumentException: The username of a Member must be different from an emptry string

Parameter name: loginName 

Exception

I think my problem might be in web-config...

is working on a reply...