Cannot remember Apple ID password created 15 years ago!

When I purchased my first iphone, I set up an Apple ID and password - 15 years ago! I cannot remember the password and contacted Apple a while ago and they helped me set up a new Apple ID and password.

They then advised me to go through all of my apps and delete them ad then download them with the new Apple ID and password.

I have a message from What's App to say that the App needs to be updated within the next 5 days. I decided to first back up my phone to ICloud as I dont want to lose all the information that I have on What's App. When I try to log into ICloud the old AppleID appears and of course I dont have the password!

I need some help please!

Many thanks