How to use the OnBeforeIBlockSectionUpdate event for my purpose?

Question

1C-Bitrix

Good day!
I know very little about Bitrix, and I learned about event handlers from Bitrix support.
There are sections on the site that I would like to remove, but leave the product (the product is linked to the section above). If you deactivate a section in Bitrix, then when exchanging with 1C, it will be activated. But the structure in 1C is built in such a way that there are many subsections and then the product, in 1C it should be left that way, but on the site it should be done differently. I wanted to change the unload tree in the exchange module, but the support advised me to use the OnBeforeIBlockSectionUpdate event. But I don’t understand what to enter there ... Do I need to somehow access these sections, enter an ID or a symbolic code? What should the code look like in general? Where to be located? Tell me please!