1. Open Eclipse and right click on your Java project, select "Properties"

2. Select "Java Build Path" and go to the "Libraries" tab as shown below.

3. Select the already available JRE library and click on "Edit" button present on right side. You will see one more new windows called "Edit Library" as shown below.

4. Here you have two options, either select any other available environment from the dropdown or if jre is not listed in the above dropdown and if you know that it is already installed in your computer then you can configure new JRE for your project by clicking on "Installed JREs..." button.

When you click on "Installed JREs..." button, you will see following window.

5. Click on "Next >" button on above window and on next windows, provide where JDK / JRE is installed by clicking on "Directory..." button (Assume that you don't have JRE 1.8 installed and you want to select JDK 1.7), Click 'OK' button and at the end click on "Finish" button.

6. Select the checkbox next to newly installed/configured JRE as shown in below screenshot and click "OK" button.

7. Last step is to select newly added JRE, from the dropdown as shown below and click on Finish...

All done, now hopefully all JRE related errors will go out from the your java project.

Let me know if you have any problem with above steps.

Thanks!