Execute in root folder:

npm i
npm run build && npm run yalc:all

Execute in current folder:

touch .env # set PUBLISHABLE_KEY, SECRET_KEY and JWT_KEY from Clerk Dashboard API keys
npm i
rm -rf node_modules/@clerk
yalc add @clerk/clerk-sdk-node # also add the packages you made changes to, e.g. @clerk/backend @clerk/types.

First, run the development server:

npm start

Open http://localhost:3000 with your browser to see the result.

Apply change in packages/* project folder and run npm run build. Then restart Express server by killing the current and executing npm start and the change should be visible.