I don't quite understand the difference. I saw a similar question on English SE, but it was a closed question (I wonder why) and the answer was a bit advanced for me to understand.
I tried to understand from a few Google results, but couldn't find anything useful to me.
Then I checked in the Oxford dictionary, which I also felt wasn't clearing the difference much. For example, according to dictionary:
The definition & example of upcoming: Oxford Learner's Dictionary
going to happen soon
Example: The upcoming presidential election
The definition & example of forthcoming: Oxford Learner's Dictionary
going to happen, be published, etc. soon
Example: The forthcoming elections
Now, seeing this, I can't really differentiate it. And I don't know how to decide which one to use.