こんにちは is pronounced Konnichwa but is spelled Konnichiha. As you take more Japanese you will see that ha is pronounced wa in other circumstances. For example the particle は is also pronounced wa as well.

こんにちは(kon'nichiha) is pronounced kon'nichwa because, when the は character is used as a particle, it's pronounced "wa". Even though it seems like a single word, kon'nichiha is a shortened sentence. kon = this, nichi = day, and ha is what's called a topic marker, stating that "This day" is the topic of the sentence.