It is not possible to delete data directly from Science Workshop except when you are collecting data using the "Keyboard Sampling" mode.
While recording in the Keyboard Sampling mode, you can delete data using the "Delete" button on the bottom of the keyboard sampling dialog box. Data must be deleted sequentially: If you have collected five data points and you wish to delete data point #2, you must first delete #5, #4 and #3.
In the Graph display, you can exclude data from analysis by selecting a region of interest. To select a region of interest, click, drag and draw a box around the area you wish to analyze.
To add, remove or modify data, you can export a data run (or copy and paste) data to a word processor, make the desired changes and import the modified data back into Science Workshop.