Where should I start with modding?

This is definitely not everything the editor offers; it's just what I remembered right now.

Thank you, the idea of using the clipboard is very helpful; I need to test it. Copying data from Excel would be great, as manually changing the numbers each time is not very convenient. On the other hand, my tables are arranged a bit differently, so I need to test it.
I understood about saving data right after the first time all the changes disappeared. By the way, you wrote it incorrectly; the editor doesn't actually modify the data.pak file. Instead, it creates files in the mods folder, and when I pack the mod into name.h5m, it works from the maps folder. If I remove my mod from the maps folder, all the values revert to the vanilla state, which indicates that the data.pak file remains untouched.

Added 10 minutes ago
I've already reviewed all the available tables and realized that a large part of the game is not accessible for editing using this method. However, overall, you can do quite a bit of work with the game. I hope the Heroes community is still active so that we can test the balance improvements later (from my perspective).
In general, I believe that you shouldn't break what already works well. However, when balancing, I encountered the problem that you need to not only try to improve a specific unit but also ensure that the overall strength of the faction remains on par with the others. Sometimes, I even have to make some units weaker, just for the sake of balance, and others stronger. I could use testers who could test hypothetical battles between factions with equal calculated strength. I've created a map for testing, but it takes a considerable amount of time to test everything. If anyone is interested, I can share the mod and the testing map. We need to check the losses incurred by each faction against other factions. Statistically, this will allow us to identify shortcomings in my work and correct them. For now, I'm only testing unupgraded creatures.

Ideally, the Barbarians should defeat everyone because they immediately have Blood Fury active from level 1 and don't use magic. However, many other races are designed not only around units but also around using magic and other unique features. Therefore, it's normal if the Barbarians and Haven are at the top.