Currently I own a lot of altcoin, I bought them at ico. My account is being downgraded several times. There are many people who say bitcoin will continue to decrease, so altcoin prices also decreased compared to USDT. So now, should I sell them and wait for the price to buy back? Give me advice 
don't sell them.. it's now too late to do that, Just wait fro the market to go bullish then you can sell them. another thing you would have done during the bearish market was maybe to trade them against USDT and wait fro the market to rise the you trade them back from USDT bt all that is now too late to do