Ivy compiler is made default compiler on Angular 9. This enables final js bundle size to decrease 25-30% improving overall app performance. Until Angular 8 ViewEngine was default compiler and rendering engine.
Ivvy compiler has improved i18n, type checking, css and style binding, debugging, improved build time by sing AOT.
You can upgrade to Angular 9 by doing
Remember that you should upgrade to Angular 8 before upgrading to Angular 9.