Hello,
I am just learning Japanese and teaching it in my Youtube channel.
I just came across Good afternoon.
Konnichiwa.
Why is (wa) cconverted to (ha) in writing here?
こんにちは not こんにちわ
The discussion of the particle は is in lesson 1, page 42, and the discussion of its pronunciation is on expression notes 2, page 46. Definitely don't miss those little notes; this book really does teach a lot through them.