Before resetting your password, be sure to confirm the email you used for your account by opening the Libre menu, then selecting "Settings", then choosing "Account Settings". The Account Settings page will show the email address associated with your account.
Once you've confirmed the email you used to sign up for your Libre account, you can reset your password here: https://www.libreview.com/
You'll first need to confirm your country, and you can exit out of the privacy choices popup if it's in your way.
You'll then see a Member Login box where you can test your login credentials. Below the login button, is an option "Forgot Password".
If you select that, you'll be able to enter your email address, and password reset instructions will be emailed to you.
Be sure to make a note of the password you create so you can enter it into the "Share Data" step of your Theia Onboarding.