Sorry for too lazy to do ''Search'' on Google / here.
1) Got some tokens in my ETH address, where can I exchange those tokens to BTC / ETH?
2) How to decide which ICO is not ''scam'' / worth to invest?
Check their site / White Paper? as there are many ICO nowaday.
3) What the difference between I invest during ICO and after ICO?
I know the price would be difference, but at least you will know is the altcoin ''dead'' or not after the ICO.
1. You got some token in your eth address.So first need to add these coin in any exchange.please follow their Ann thread to know about listing any exchange or not.if coin added on exchange then you can exchange these coin by BTC. If you have only eth coin.you can exchange these on bittrex.
2. Yes for knowing about the Ico need more research. And you may take a good dicission by the manager.If manager have green trust or your known person.then the ico will be succesfull and you can invest during ico.
3. in ico investment is risky but if your luck is good you will get more benifits after ico.And they send some extra reward during Ico.
I think this answer can help you.