When do I give the Out signal, and can a decision be changed?

Appeals and the Out signal. Under Law 31.1 an umpire does not give a batter out without an appeal. Cricket Australia's Community Umpiring Technique Guidelines say the Out signal (raised index finger above the head, Law 2.13.1.1) only needs to be given for appeals relating to LBW, caught behind (batter stands their ground), run out, stumping, obstructing the field, timed out, and any case where the batter stands their ground. It does not need to be given when the batter immediately "walks", for example bowled, an outfield catch, or caught behind or in the slips. If the batter is not out, call "Not out" (Law 2.13.1.1).

Doubt. The guidelines' LBW process ends with "if you have ANY doubt" the decision is Not out. Under Law 31.6, if after consulting your partner on a point of fact there is still doubt, the decision is Not out.

Changing a decision. An umpire may alter any decision provided it is done promptly and does not contradict Law 20.6 (dead ball not to be revoked). Apart from that, a decision once made is final (Law 2.12). The umpires together make the final decision on any disagreement or dispute (Law 2.11).

Withdrawn appeals and wrongly leaving batters. The fielding captain can withdraw an appeal only with the consent of the umpire it falls under, before the next delivery (Law 31.8). If a batter who was not given out leaves the wicket under a reasonable misapprehension of being out, either umpire calls Dead ball and recalls them (Law 31.7).