How can I update a lot of scores for one student?

If you have a stack of make-up assessments from one student, you can use Gradebook to quickly enter those scores. Locate the student in the correct Gradebook (i.e. the right course, section, and grading term) and then click the pencil icon to the right of his/her name.

In the window that pops up, you can move from assessment to assessment for just that one student. You can grade his/her assessments using the same Gradebook workflow that you use to mark different students on a single assessment: set a percentage score, mark as "Missing" or "Excused", fill down, and comment on specific results.

Use your keyboard's tab key to quickly move from field to field.

If you were viewing Gradebook in "Timeline" view when you clicked into a single student's results, you'll see all assessments listed in the order in which they were displayed on your screen. The same goes for "Bucket" view, which is to say that assessments will be organized first by methodology bucket and then by date. If you were viewing Gradebook in "Standards" view, you may see fewer assessments than you expect. This view is limited to single-question, 100-point assessments due to the complexity of standards-based grading. To edit results on assessments that don't show up, you can view the full assessment.

Lastly, be aware that the assessments you see will be limited by the grading term you've selected. If you have several Q1 assessments to grade, and several Q2 assessments to grade, those will have to be done in two batches.