If you're coming from 2.x, you should first checkout the 2.x -> 5.x migration docs
Migration from 5.x to 6.x
The main change between 5.x and 6.x is that in version 6 the Toolkit is shipped as a single package - @jsplumbtoolkit/browser-ui - which contains all of the code that in 5.x was spread across a number of packages.
Library integrations - @jsplumbtoolkit/browser-ui-angular, @jsplumbtoolkit/browser-ui-react, @jsplumbtoolkit/browser-ui-vue2, @jsplumbtoolkit/browser-ui-vue3, @jsplumbtoolkit/browser-ui-svelte - are still shipped as separate packages, but note that their related -drop packages are now incorporated within the library integration's main package.
Spring Layout
This has been removed in version 6. You can use the ForceDirected layout instead.
Templating
Templating now uses the {{someVar}} syntax for variable interpolation; the ${someVar} syntax is discontinued. This also means that expressions (such as ${someVar * 2}) are no longer supported, but you can use template macros for that sort of task.
Renamed methods
As a result of the changes to packaging, a few methods were removed/renamed. A full list of these is available in the Changelog