See this, you might get Idea: https://nextjs.org/docs/app/api-reference/file-conventions/metadata/app-icons

just replace the favicon.ico in yout app folder. Nextjs will use it automatically.
If you have a png or other sources and you want to create a favicon.ico you can use generators for that:
https://www.favicon-generator.org/

btw, browsers cache favicons heavily, so even if you change it, it may take some time to how (unless you force refresh)

@West African Lion have you solved your question yet, or do you need more help?

Devtools (inspect element) -> Go to Network Tab -> Disable Cache. Would probably work to see if the favicon is working