The A in Agile is from automation. At least in the context of Agile development.
Automate everything that you can. Start with that. It may be a bit slower to do the first time, but as Agile development is iterative, it will likely gain the lost time in no time. It enables scaling of organization and products. It enables being fast and delivering with quality.
Automation reduces being dependent on individuals. In a good way.
To be able to react on changes, create the best product version for that moment, test it good enough, and deliver it is possible manually. However it may be slow, unpredictable, unreliable, and fail because of a flu.
Automatize it, if you can. Especially regression testing.