CREATION:
new string[128];
format(string,sizeof(string), "%s", PlayerName(playerid));
namelabel[playerid] = Create3DTextLabel(string, COLOR_WHITE, 0, 0, 0, 40.0, 0);
Attach3DTextLabelToPlayer(namelabel[playerid], playerid, 0.0, 0.0, 0.2);

UPDATE:
format(string,sizeof(string), "%s", PlayerName(playerid));
Update3DTextLabelText(namelabel[playerid], COLOR_WHITE, string);