There are three main study guides available to upgrade radio license. The American Radio Relay League and Gordon West both publish study manuals with the full question pools and explanations. Others, such a Dan Romanchik, also have shorter study guides. These resources are all linked below.

ARRL Extra Class License Manual

Gordon West Technician Class Manual

KB6NU No-Nonsense License Study Guides

Additional resources

YouTube features several video license classes, including an in-depth series based on the ARRL License Manual by David CaslerHamStudy.org provides practice exams and flash cards for learning the material.