To obtain the latest release, include the <version> in your POM:
<plugin>
   <groupId>org.apache.maven.plugins</groupId>
   <artifactId>maven-foo--plugin</artifactId>
   <version></version>
</plugin>

If you have omitted the version, you can update to the latest release 
using the -U flag:

mvn -U <phase> | <goal>

- The Maven Plugins Team