Harry: Agreed, when I took my exam in 2018 there were only a handful of questions about agile.
However Arnaldo, from what I read about the upcoming exam change in July, there will be more questions about agile. I would at least read the PMI Agile Guide at least twice just to be prepared.
For starters, learn about the difference between traditional waterfall, hybrid and agile projects. Retrospectives. Backlogs. Kanban, etc.
Cheers,
Joe