Designating an heir is a good idea. Death being final and all.
To do so you must satisfy a couple of preconditions. You have
to be in a location which includes a Notary's CityNet terminal.
And you have to have in your inventory a Notary's certificate.
You've already figured out that doing these things requires
a Notary.
You can change your designated heir as many times as you like.
Just use the Will command again. Each subsequent use updates your
designation with your new choice.
Make sure to keep your will current. If your designated heir is dead,
your will won't be useful. Stuff like that.