There are two ways to update ScriptRunner for Confluence: 

Version History on the Atlassian Marketplace

To upgrade ScriptRunner for Confluence, navigate to Version History. From there, you can select a version to Download

You can also click Watch in the upper right-hand corner to be notified when new versions are released. 

Manage Apps in Confluence Instance

If you navigate to General Configuration Manage Apps, you can update by selecting Update