Starcraft 2 Changeling tactics are sometimes not easy to pull off. In this real time strategy game, time and resources are everything. It's possible to use these units in a couple of different ways if a player is willing to micro manage them along with their Overseers.
A changeling is a Zerg unit that is spawned by the Overseer. It is a weak and short lived unit. It has the ability to mutate into a nearby basic enemy unit, meaning that it can almost effectively blend in with the enemy's forces.
Because of the micro management involved with this unit, many people refuse to explore the options these units have. Changelings or clings require an Overseer to be created. They are typically dropped near enemy forces, which means they might cause an Overseer to become exposed and vulnerable to attack by sheer proximity to the enemy. There are ways to minimize this risk.
Against a Terran enemy, it's best to drop these units at the perimeter of a base or enemy army in order to slip them in as covertly as possible. When observing enemy army movements it's best to get the cling to follow a Marine unit so that they blend in with the rest of the army easier.
The psionic impressions that a cling gives off make it so that enemy units do not find it aggressive. It is not killed on command like other units might be. It won't be attacked unless the enemy player notices it. A possible way to use clings is to make a shield out of them. This is especially effective against Protoss and Zerg enemies, but will not last for long.
Against a Zerg enemy, it's best to drop clings behind the enemy's hatchery. The main goal of a cling is to give information about the player's base, expansions and technology or tech. Small changelings are hard to click on if they're stuck behind the hatchery structure. Even if they are noticed, the enemy player may have difficulty clicking on them to get them destro[censored] , especially if they keep moving.
It's more difficult to use a changeling against the Protoss. This is because many Protoss players do not use many zealots later in the game, and the main form for Protoss that a changeling can turn into is a zealot. However, that doesn't mean that it's impossible. The cling can still explore the Protoss player's forces temporarily and allow players to see into the enemy's bases, even if only for a brief moment before discovery.
The best way to actually defeat Changeling tactics is to watch your own units carefully. If you suspect that a Changeling is hidden within your forces you should try box selecting your army and checking whether any units are not selected. You'll then need to use the attack command to manually kill each Changeling.