Changing Java Version From Within Eclipse

I wrote a project in eclipse with an older version of java. Now on a different computer but running the same codes I'm getting a whole lot of red. This is due to Version incompatibility. I've had my professor change the Java version from within the console window before. The code compiled just fine after he did so. I'm sure its the same case with this code. After looking at other tutorials and google links I could only find command line approaches and @override methods. To be honest I still don't understand these. What is the best way to change the version of a code originally written in an older code inside the eclipse console? thanks!