Choose 'Pupils' on the left-hand menu. You can search for the relevant pupils by entering name, class, username etc into the search box.
Highlight one or multiple pupils by clicking on them.
Click on the orange edit button.
If you have only selected one pupil you will see a full range of options you can edit.
If you select multiple pupils you will see fewer options as edits here will apply to all selected pupils. For example, you can't multi-edit usernames as these must be unique.
When you have finished making your changes, click save.