To edit layers within a map, make sure you are in the Map View of whatever map contains the layer you want to edit. Head to the Hamburgeler (menu) icon next to the layer you want to edit, and select Edit Layer.
This allows you to select individual features, delete them, edit the attributes, change the location, or change the geometry of the feature. Don't forget to save your edits!
To edit the layer's attributes directly, go to Edit Layer, and click on the Feature Attribute Table. From there you can edit any of the information in the fields as well as add text, and once you save the changes will be automatically associated with that layer.
Please note that changes made saved are final, meaning that if you were to share the layer/map in the future or download it such changes will be reflected.