You’re dealing with an altogether different framework based on the concepts that AngularJS introduced. It is likely there is no simple path to simply upgrade the existing application. However, before you give up and start from scratch I would check out the upgrade guide published in the Angular Docs and see if ngUpgrade can’t be used to help you migrate your AngularJS application. This library can aid in migrating most small-to-mid-size applications.
No matter which path you take you will have to get familiar with Angular (2/4/5/6/etc) to make informed decisions on how to proceed at each step.
There are benefits to taking this as an opportunity to start fresh and it is probably more maintainable to do so in the long-term, but depending on your situation that may not be practical, so you’ll have to assess the options and determine which one seems most appropriate.
