On the application login screen, click "Forgot your password?"
Enter the email linked to the account;
Click on recover;
Check your mailbox to find the message. Click on "Reset";
The link will take you to a site where you can change your password and confirm the latter (enter your password twice).
If the "reset" button does not work, you can always copy the address located just below (in blue) in the URL bar of your browser to go to the same place.
If you don't get the email, check:
Is this the correct email (the one linked to your Hexfit account)?
Have you looked into your spam (SPAM)?
Make sure the email was written correctly when retrieving the password, otherwise it will not be able to reach your address.