Answer by Pankaj for How to set my default shell on Mac?

1. Open your terminal and press command+, (comma). This will open a preferences window.
2. The first tab is 'General'.
3. Find 'Shells open with' setting and choose 2nd option which needs complete path to the shell.
4. Paste the link to your fish command, which generally is /usr/local/bin/fish.

I am using macOS Sierra.