Follow the below steps to change the default index page in Parallels Plesk Panel 11.5 for Windows:
1. Login to Parallels Plesk Panel
2. Click the domain that you wish to change.
3. Click on “Websites & Domains” link.
4. Click on “Show Advanced Operations” link from the bottom of the page (You can see like one arrow pointing to down)
5. Click on “Virtual Directories” icon.
6. Click “Directory Properties” icon.
7. In the “Documents” section, find the “Default documents search order” field. You can add new default file names and change the order of precedence for the file names.
8. After finished making changes click the “OK” button at the bottom of the page.