One way to go on about this is to make a list with all your Exchange accounts and put them in a bank locker or with your lawyer, to hand to your people when you pass away. They will now require to use the usernames and the official death certificate to reset the passwords with the exchanges.
This should work fine with the big exchanges as Bitfinex, Coinbase and so on. But some smaller, and some foreign exchanges might make problems, requiring further documents, or rarely replying to the support requests from your relatives. There also is a tiny risk that your relatives would forge a death certificate to get immediate access, like in a divorce or falling out.
Another way is to create a word seed for your data and give half of it to your relatives, and put the other half into your testament.
An easier way would be to use the services of bitcoinforeveryoung.io as we would also be glad to help your relatives with accessing the values when they are not that tech-savy, to make sure they receive what you have planned out for them.