No worries, it can happen to the best of us! Follow the instructions below carefully:
Note: Be sure to have your original Sky account saved first with an external account like Google/Apple ID/etc. before starting this process!
Go to your mobile device and make sure you have updated Sky
Start up Sky, choose ‘Sign In Method’ at the bottom
Log in with your Steam account that you accidentally linked your Sky PC account to
Go to the top right-hand corner in-game, hit the cog wheel for options
Go to Account and link it to an extra or spare Game Service account (e.g.: another Gmail account) that is not associated with your original Sky account
In the same Account window, hit the icon next to your Steam ID to unlink it and confirm
Restart Sky, and choose ‘Sign In Method’ again at the bottom
Log in with the appropriate external account your original Sky account is attached to
After you verify you are back on your original Sky account, go to the top right-hand corner in-game with the cog wheel
Go to Account, and link your Steam account properly
Congratulations, you are good to go and play Sky PC!
If you need more help or information with linking your current Sky account to a new platform, please check out this link: https://thatgamecompany.helpshift.com/hc/en/17-sky-children-of-the-light/faq/703-how-do-i-save-my-progress/?s=my-account&f=how-do-i-save-my-progress&p=all