WeBWorK keeps a spreadsheet with homework scores for you. When you score a set, you are adding a column to that spreadsheet with current scores from a particular set. If you then give someone an extension, you may want to score the set again. This will not replace the existing column in the spreadsheet; it adds another column for the chosen set.
Scoring a set
Scoring Tools
from the left panel.
Score selected sets(s)...
Downloading the scores
Once you have scored at least one set, you can see the spreadsheet on the scoring page, both rendered on the page and as a link. Click on that link to download the file of scores.