Have you ever used a kitchen faucet and when you turn the hot water on, cold water comes out instead? You usually find this after someone has just installed it or repaired the faucet. There a couple of situations that could have caused this.
The first situation is where the installer actually crossed the water supply lines. You will see the hot water supply line connected to the cold side of the faucet and vice-versa. I can’t say this is very common because the hot water supply valve is under the hot side of the faucet. In other words, the left side of the faucet is the hot side, and the left angle valve is the hot valve.
What is more common is that the faucet is a single handle faucet and the cartridge’s stem just needs to be rotated 180-degrees. To access the stem, pop the cover off of the handle and remove the screw. This will allow you to remove the handle. The stem is what the screw was removed from. It is usually silver and sticks up noticeably. Hold the stem with two fingers and rotate it 180-degrees, then reinstall the faucet’s handle. That should solve your problem and make the right side cold and the left side hot.