Continuous improvement is not an option

An agile team's most distinguishing characteristic is that it continuously improves. Getting better is not unique to agile teams. After all, non-agile teams can improve as well. For an agile team, however, continuous improvement is not an option. It is a defining attribute. A team that is not improving is not an agile team. 

At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly.
— 12th principle of the Agile Manifesto,

The team determines its improvements

How the team improves, in what ways, and how much are determined by the team itself. Imposing improvements on a team doesn't work. Oh, it might... for a while, but an imposed improvement will only last as long as the imposing force remains... and the team puts up with it. For an improvement to become a natural, sustainable behavior of the team, the team must decide for itself that the improvement is needed and do the hard work to make the improvement a default behavior of the team.

Continuous improvement is the key default behavior of the team

A one-time improvement may be a good thing, but agile teams don't stop there. An agile team constantly challenges its status quo, embracing on-going experimentation and self-reflection to discover improvement opportunities. Continuously seeking and implementing improvement is the key default behavior of the team. 

Beyond improvement for improvement's sake

The best agile teams are intentional in the focus of their improvement. These teams reflect on their current state and identify the improvements that will have an optimal positive impact. Just as product owners focus teams on the most valuable work, the best agile teams focus on improvements that have the biggest impact on their effectiveness.

How has your team improved lately?