Hopefully, you aren't surprised by this very agile answer:
Start small.
Demonstrate value.
Measure, inspect, and adapt.
Iterate out and up.
Find and train your Product Owners and develop their skills.
Create a backlog and prioritize it.
Have newly trained Product Owners pick items from the backlog, and practice splitting stories and defining acceptance criteria.
Perform your own retrospectives.
Request regular product progress demonstrations from the development teams.
Find and train your Scrum Masters and develop their skills.
If the business has not provided a backlog of known work, create one and practice relative sizing.
Establish a fixed cadence, practice the Scrum ceremonies, and invite the business to regular demonstrations of product progress and value.
Integrate writing and automating unit tests into your expected development practices.
Set up a continuous integration framework.
Perform your own retrospectives.
Find or create Communities of Practice of relevance to your direct reports.
Support your team in getting trained in an agile framework.
Get trained in the agile mindset and framework to understand what your teams must do and what they need from you.
Promote agile values and address agile roadblocks.
State a strategy to tie features and workload directly to executive vision, and inspire and align your teams to it.
Find and support a guiding coalition of agile champions.
Recognize that implementing agile is not a technical challenge; it is one of organizational change.