"こんにちは" is the correct way to write it. "は" is used instead of "わ" because originally (in ancient/old times) it was used as a part of a sentence: "こんにち は ~ / Konnichi wa~ / Today is~ ". "wa" is a particle in that context that's why it was written as "は". After many years, it was then adapted as a fixed greeting. That's why it retained the character "は". Hope that helps (^_^)

Take it as an online-related slung that is prevalent among young generations. こんにちは is definitely correct but sometimes seems too polite so that they intentionally twist it into こんにちわ. In Japanese, there is few to express "Hello" in a casual way. So it's pretty understandable.

Although the government's guideline says that こんにちは is correct, people occasionally use こんにちわ either in jest or just out of ignorance. Apart from that, わ may be more appropriate for abbreviated greetings like こんちわ, ちわ, or ちわーす when writing in a conversational tone.

The correct one is こんにちは, but some Japanese people write it as こんにちわ when chatting.