Hello Raul,
You can move atoms by going to 'Edit | Edit Data | Structure parameters...' and changing their x, y and z coordinates in the list.
If your atoms are related by symmetry, however, moving one will move all others of the same type. The way to get round this is to go to the 'Unit cell' tab and clicking 'Remove symmetry'. Now, on the 'Structure parameters' tab, each atom should appear on its own line and you can move one without affecting the others.
Hope this helps,
Philip