This can be done in multiple ways.
Let me list down steps for the simple way.
1) Right click on the project and choose Properties option.
2) Select Java Build path and click on Libraries tab.
3) Remove JRE System Library [1.4....].
4) Click on Add Library... button
5) Select JRE System Library
6) On the next screen, select 'Alternate JRE' option. The drop down will list all avaialble JRE libraries in the stytem. Choose one of those and click finish.
You will see the JRE libraries added in the libraries tab screen.