Student work in Perusall is graded automatically, but you always have full control and can override the annotation scores provided by Perusall.
In context: Open up the assignment and then you can see all of the highlights and conversations as students see them. To see the individual comment score assigned to each comment as you are reading, click the filter dropdown menu (which says All Comments by default), and select Show comment scores. You will then see a score for each comment beneath each comment bubble, and can then change a score from Perusall's automatically-generated score by clicking on the score and selecting a new score from the dropdown.
In a spreadsheet: If you would rather review the comments in spreadsheet format, go to your course dashboard, select Assignments at the top, click on an assignment, and then click All comments. Individual comment scores will show for each comment in this view, and you can change a score from Perusall's automatically-generated score by clicking on the score and selecting a new score from the dropdown.