Help with new Mac

[Fionn]

I recently bought a Mac second hand from my friend who i'll call Brian, my name is Fionn. When I go into Macintosh HD it shows a picture of a house (I think its home) and has Brians name there. I went change it to Fionn which it did succesfully. But after a while it changes back to brian and shows a folder icon instead of a house?Any help?

 

[simbalala]

Change it back to what it was.

Then using that account create a new user for yourself with admin privileges.

There are other options but they can get complicated, do the above first.

Edit: Here's one article - http://support.apple.com/kb/HT1428

Also read: http://www.danfrakes.com/ , the ChangeShortName section. Pay attention to which version of OS X you're running.