If you are using Maven
Add the compiler plugin to pom.xml
under the top-level project
node:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>
(Hoisted from the comments.)
Note: If you don't mind reimporting your project, then the only thing you really need to do is change the pom and reimport the project, then IntelliJ will pick up the correct settings and you don't have to manually change them.