How to shoot 800asa film as 400?

Edit: who am I pissing off and why they downvote my post and comments?

Hey all,

I purchased 2 kodak portra 800 and I decided I prefer to use them as 400 asa instead for daylight or brighter interiors.

What are the correct settings to put in order to do that?

Do i put 800 Asa and underexpose a bit? Or something else?

(I also have a Cinestill tungsten 800 but not sure if the colours will turn ok during day)

Thank you so much!