Create a New Marker. Left mouse click to store a new lat/long in the Data Entry section. Edit the fields and click Add Marker. The Data Entry area always shows the top entry in the table.
Edit a Marker. To fix a typo or move a point etc. Right click on a marker. The Data Entry section will show the details for you to edit. Left click on the map to adjust location. Add Marker to save the new work. This creates a copy (just in case you've made a mistake). Look at the Data Entry Table, the top version is the new one. Delete older markers (red button with a trashcan icon). It's easy to forget to delete the old one, as they may be in the same place if you were simply editing text information.