The sveltejs/template uses sirv-cli.
You can add --port or -p in your start:dev script in package.json.
Instead of:
"start:dev": "sirv public --single --dev"
Use:
"start:dev": "sirv public --single --dev --port 5555"
You can see more of sirv-cli options:
https://github.com/lukeed/sirv/tree/master/packages/sirv-cli