To edit or delete an existing record (contact, company, task, product, etc.), you must have editor or owner level access on that record.
Select the item you want to edit from a list and view it.
If you have edit rights on a record, you will see an edit button at the upper right corner. Click on the edit button.
Make the changes you want and click on the save button.
If you want to delete the record, press the Delete button on the bottom right corner.