To remove a negative keyword from the Squared: Account Negative Keyword List you'll need to head to 'Protect' and 'Negatives' in the platform. From here you can filter for the negative and click the red 'x' to remove.
If you can't find the negative here, try the Group or Global Negative lists applied to your account.
If you remove a negative in the shared library list directly in Google Ads, but not in Squared.io, Squared.io will reapply the negative at the next sync.