If you have Teacher permission and are assigned to the class, you can go to My Subjects, choose the class, select Baseline as the Checkpoint and then change the mark entered here for the individual student.
If you have Admin permission on ScholarPack, head to Admin>Students>Assessment Checkpoints. Enter the Academic Year, the Subject, Student Year Group and Class, plus Checkpoint Baseline. The list of students in that year/class will come up, with existing baseline marks. You can amend any here, and then click on Save at the bottom of the table.