If you wish to edit one of the interventions you created earlier, you go to the "interventions" tab in the detailed view of the asset and choose the intervention you wish to modify.
A pop up will then appear. Click on the "edit intervention" button at the top of the pop up to edit the intervention. Click on the "update intervention" button to save your changes.
As a Railfleet admin you can also modify interventions (or search for old interventions) in the administration panel.