Below is archive link which has reference on how to set up raspberry pi 3B+ in headless mode. It will will also help in setting up ssh and wifi if required.

• https://web.archive.org/web/20191227222211

To update the default password:

• ssh to rpi using instruction above
• Use passwd command to update the password or add your ssh public key to ~/.ssh/authorized_keys folder.

To add ssh keys for login: You can also your public keys for ssh, which give seamless way to login without password hassle, but then you need to have ssh keys to login.

• Create a $HOME/.ssh folder: mkdir ~/.ssh
• Create a file called authorized_keys: touch authorized_keys
• Add you public key to file: cat <pub_key> >> authorized_keys

Further reading:

• How does .local work: https://en.wikipedia.org/wiki/.local
• What is mDNS: https://en.wikipedia.org/wiki/Multicast_DNS
• What does avahi daemon: https://en.wikipedia.org/wiki/Avahi_(software)