There are two ways to do this.

1. From the Encounters -> Assessment screen, you can always go to the "Pt Hx" tab to view and select any of a previously used diagnosis (ICD-10) code for this patient:

2. If you want to use a previous encounter, you can use the "Import from" a previous encounter by keeping your cursor on today's new encounter and the click with your right-mouse button on the previous encounter to be able to see and select "Import from"

You will be presented with a list of default encounter summary sections and here you can just select the Assessment section:

NOTE: Any section you select for importing WILL OVERWRITE ANY DATA PREVIOUSLY ADDED TO THAT SECTION.