You can sign into Flood using your GitHub account by clicking the "Sign up with GitHub" option on the signup page.
After you have signed in to your GitHub account, you'll be redirected to Flood and prompted with this popup:
Sign in to Flood using GitHub account
If you signed up for Flood with a traditional way (using an email and password) but later realised that your email is also used in your GitHub account, you can sign in to Flood using your GitHub account by clicking the "Login with GitHub" option on the sign-in page.
Follow the prompt to log in to your GitHub account. After authenticated, you'll be redirected to Flood.
Note that whether you sign in using GitHub, or using email & password as a traditional way, your data, identity and everything else within your account will still be the same.