How Do I Change "unload" to "visibilitychange" ?

I am hopping in way over my pay grade here.
I know that I should be posting this on the paid support forum.
Just hoping that someone can guide me on the safest way to make the code changes mentioned in the title.

After extensive testing and site improvements, I have been able to speed up site load times dramatically.
I am seeking to fine tune my loading environment to help speed up the mobile side of things.
Light house is flagging my site for using the "unload".
Best Practices Score of 87.
The recommendation (per Lighthouse) is to use the "visibilitychange" instead of "unload".
If anyone reading this has had success with this I would seriously appreciate a little guidance.
Wiling to do the work, I just do not know where to paste the code, or even if that is all that I need to do.

https://www.personaldronerepair.com
Thank You !!