This is just a soft fork or a hard fork? I mean the coins will be totally replaced with Ycash? And should all Zcash withdrawed from exchanges before fork or can stay on exchanges?
This is a hard fork, similar to the BTC/BCH and BCH/BSV hard forks. Ycash will be a new coin that shares the same ledger as Zcash up until block height 570,000.
The only exchange thus far that has announced that they will credit ZEC holdings with the equivalent amount of YEC is Safe.Trade. Unless your exchange explicitly says that they will credit ZEC holdings with the equivalent amount of YEC, you must assume that they will not do so.