updating child version of theme

Hi,

I use a child version of Highend theme. The version I have is 1.0.1.

Can you help me understand the steps I need to do to update to the current theme version with particularly regard to updating the child version?

I don’t remember how but in the past I must have downloaded 2 newer versions found in the theme section but I never activated them because I was afraid of losing any custom work that I did.
3.4.9.1 AND
3.6.2