How can I update my iCloud?

This is a pretty old story, but I think it's still relevant. Some years ago I bought a new iPhone because my previous one had gotten wet. When I started setting it up, I couldn't transfer the data from my previous phone to the new one because it didn't turn on. What I did was transferring it from my iCould. The problem was that the backup was from a long time ago, so it wasn't up-to-date (I had old WhatsApp groups, my old wallpaper, old apps, etc.). Obviously, after that I had fixed my old phone and transferred the data from it.

But I'm still wondering what would've happened if I had lost my old phone. I backed up my current iPhone to iTunes on my PC, but I'm still worried. What if something goes wrong? What if I don't have my PC?

What can I do to make sure that I will be able to transfer my data to a new iPhone?

Maybe I should delete all of my current iCloud to back up new stuff? Won't it delete what I currently have on my phone?