[M4D]

because you just format mysql query and you don't send it to database !
you have to mysql_query() function.
i edited your command

pawn Код:

CMD:password(playerid,params[])
{
  new q[126], str[100], newpass[100];
  if(sscanf(params, "s[100]",newpass)) return SendClientMessage(playerid,Red,"Chnage password: /password <newpass>");
  pdata[playerid][Password] = newpass;
  mysql_format(Mysql,q,sizeof(q),"UPDTAE `players` SET `Password` = %s WHERE `ID` =%d",pdata[playerid][Password],pdata[playerid][ID]);
  mysql_query(Mysql,q); // <--------- Here
  format(str,sizeof(str),"Password has been change to <%s>",newpass);
  SendClientMessage(playerid,Red,str);
  return 1;
}

an important note: please make a permision for using this command !
for example if player logged in successfully then he can use this command
if you don't do this:
i'll join your server with an admin nick
i'll press F6 before login and /changepass
so before loging in i'll change admin password and now i can enter server as admin !!
Heed buddy