Jiří,
Welcome! You've found the right place to ask these kinds of questions.
First, if you are using the switches as locally managed switches then you control that yourself. You will need to download the updates manually from the Instant-On site.
If you are managing the switches via the Instant-On Portal, software updates are done on an automatic schedule per site. I think the easiest way to manage that would be to create multiple sites and fan out your switches into the additional sites.
A "site" can mean whatever you want it to mean. It's a way of segmenting your network devices.
Once you do that, go to the Site Software settings for each of the new sites and set the day of the week and time when you would like the updates to occur.
It's pretty easy and works very well.
Regards,
David
------------------------------
David Fox
------------------------------