Basic question (sorry for being dumb!) installing vs updating envato theme

I got my site set up with the help of a friend and did a little css and customization myself a bit afterwards, this was 5 years ago. I went through a divorce 2 and a half years ago and haven’t updated my site since but want to get started again. Envato sent me an email saying my envato themeforest theme (salient) had been updated. So I downloaded the zip file… and now I don’t know what to do.

I go to wordpress and it says I can customize my current salient theme, or add new theme. I just want to update theme, not sure either of those is what I want?

Or in wordpress do I go to the salient section and go to import / export? (I am very afraid that the “import options” section here has a warning saying that this will overwrite all existing values–what will that do to my site if I’ve done some editing to it??!)

Aggg help!!