The Book "Soothsaying for Dummies" isn't giving me the option to unlearn my specialization
Battle for Azeroth: You can drop your current specialization by opening the Profession tab in your Spellbook and unlearning Gnomish/Goblin Engineering. After that, you can learn the other specialization from an Engineering Trainer in either Orgrimmar or Stormwind.
WoW Classic: To change your Engineering specialization in Classic, you must follow these steps:
Complete "Show Your Work" for your given specialization