Orangecrush's answer is technically correct, but relegates the most important point to the final paragraph: no batsman can ever be given out unless there is an appeal from a fielder. To quote from Law 27:
Neither umpire shall give a batsman out, even though he may be out under the Laws, unless appealed to by a fielder.
Hence it is necessary to appeal for all dismissals, whether they be bowled, LBW or timed out. (Note that Law 27 does not forbid a batsman walking).
Referring to the specific comments in your question:
It is the duty of an umpire to give the decision as OUT when the ball running to the stumps, hits any part of the batsman other his bat.
As noted above, this is incorrect. The umpire should not give a batsman out unless an appeal is made.
If the bowler or any of the player of the fielding side did not appeal for the dismissal, what can the umpire do in this situation?
The umpire should do nothing and wait for the fielding side to deliver the next ball of the over (or call "over" or "time" as appropriate if it was the last ball of the over/session).