Disclosure: I work for Amazon, and in the past I worked directly on EC2.

From the FAQ: https://aws.amazon.com/ec2/faqs/

Q: How does EC2 perform maintenance?

AWS regularly performs routine hardware, power, and network maintenance with minimal disruption across all EC2 instance types. To achieve this we employ a combination of tools and methods across the entire AWS Global infrastructure, such as redundant and concurrently maintainable systems, as well as live system updates and migration.