I am confused about 15A and 20A current.

Take the 15-amp circuit breaker out of your box and replace it with the same brand of breaker that's 20 amps. Email if you need directions for the removal and switch.

Unless your wiring is a minimun of 12 gauge (14 gauge is smaller) you cannot just change the breaker to a larger size. If you have 14 gauge wiring on that circuit try replacing the existing breaker with the same brand 15 amp breaker as breakers can wear out over time. If you still have a problem after the replacement, you're going to have to find another circuit with more reserve capacity or install a dedicated circuit.

You need to have a circuit that's capable of 20A as well as a breaker otherwise you'll end up defeating the point of having a breaker. While I'm pretty sure that standard 12/3 Romex is good for 20A, I'm not qualified to say for sure.