That’s called a “parapet” and I can see why it needs to be repaired.
You’ll need “form boards” installed along the inside and outside, plus you’ll need to install a strip of foam to seal the board to the wall, so nothing runs down the wall. Set the form boards level with the top of the existing wall to give you that “square and matching” look you want. Use “form ties” to hold the outside form tight to the inside form.
I don’t see any rebar sticking out of the existing concrete wall so you’ll need to install 2 - 1/4” anchor bolts about 24” apart plus’s 1 - 1/4” anchor bolt in each end of the broken out portion. Then, fasten a 1/4” rebar to each anchor bolt with wire.
Now, you can buy “pre-mixed” concrete. It needs water for the cement to “react” and for it to flow around the rebar. Let it set for 3 days before you remove the forms. (I’d use form release agent, but it’s not necessary, if you remove the forms at day 3.)
I’d use a bonding agent to make sure the new concrete seals to the old too.