How to Play TriadCity
The TriadCity Players' Guide
Tips & Tricks: File a Will
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.
Once you're ready, use the Will command.
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.