Update ENFOLD not possible

Dear Support Team,

I am a web designer and currently only use the ENFOLD theme for my clients, and I’m really excited about this powerful theme.

However, I have now with a customer for whom I finished the website last summer and just made a small Aktuaulisierung, the problem is that I can not update the theme.
The following message is displayed:
“Update Available!
A new version (4.5.3) of your theme is available! You are using version 4.0.7.
Do you want to update? ”
When I click Refresh, it does not work and I get the error message “Refresh failed”
The customer’s website runs on the PHP version 5.6.39.

Now I have looked at other customers whose website I have created in the same period, also here I have the same problem.

How should I continue here?
What is your recommendation: Is not it necessary to update the theme permanently, so that we are always up to date?
What would happen if I do not do this? Does the entire WordPress page then stop working someday?

Now I would like to make this update for the previously mentioned page https://stefanhunstein.de/. Could you please take a look at the site and take the necessary steps to be able to update again?

Basically, I still have the question of how I should proceed in the future: Does it make sense for each customer after a certain period (for example, 3 months – or what do you recommend?) To log in the backend to the ENFOLD – update to take and thus the connection not to miss the latest version?
Or is the whole thing with the update not necessary?

I am very grateful for your well-founded information here, as this topic repeatedly presents itself to my clients.

Best regards Diana Steib