How can you change the reputation in WOW?

Reputation is very similar to experience. Reputation is divided into a number of different levels for which players must earn reputation points to progress through. Higher reputation levels generally require more points than the previous level to progress. The exception is hated, which spans a very large number of reputation points. This is the lowest Reputation Level possible. This is the level below Hostile. Any player who comes close to anyone of this faction will be attacked on sight.

For example, players start off with the Brood of Nozdormu, Netherwing, and Syndicate factions at hated. Both factions start off at 0/36000, the lowest reputation possible in the game.

Reputation points, just like experience points, are gained through quests and killing various mobs. Unlike experience, it is possible to lose reputation points with some factions, either by killing members of the faction or by assisting rival factions. For rival factions, gaining reputation with one will normally decrease reputation with the other, but at a greater rate. For instance, if you kill a magram centaur, you will gain 20 points of reputation from the gelkis, but will lose 100 points from the magram. Currently there are 5 of these rival-faction pairs: Aldor (Draenei) vs. Scryers (Blood Elves), Gelkis vs. Magram (both Centaur), Steamwheedle Cartel (Goblins) vs. Bloodsail Buccaneers (pirates), Ravenholdt (rogues) vs. Syndicate (Thugs), and The Oracles (Gorlocs) vs. Frenzyheart Tribe (Wolvar).

Various quests and faction-specific rewards are available across the levels, but the player must be able to interact with the faction NPCs to gain access to these rewards.

You can gain or lose favor, otherwise known as reputation, with many of the several different factions in Azeroth by completing certain quests, or killing certain creatures. Doing so will usually unlock special rewards or new quests to accomplish.

You can also increase your reputation by doing repeatable reputation quests.