Can I copy a existing coins source code, edit it a bit, create a whitepaper and launch a ICO for my coin to collect essential funds for creating the phone app, wallet, marketing, etc.
I can't say I know a lot about it but since bitcoin is open source you definitely can do this and most of the original altcoins like litecoin, feathercoin, dogecoin and many others were like this. They were basically copies of bitcoin with slight modifications.