Just got it... Commands listed by Steffen_Volgmann worked perfect. Thanks Steffen!
My Mac... MacBook Air (13-inch, 2017)
open terminal
sudo -i
enter root password
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil
Unenroll from the Beta Program
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll
Enroll in the DeveloperSeed Beta
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed
Close and re-open System Preferences > Software Update
The new Beta should now show up
Carlos_Augusto
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