You can import Gradebook .csv export files back into Canvas after making appropriate edits. It is important to note, that if you delete or add any columns or rows into the .csv file, you will not be able to import the file into your course. All files that are exported need to only have the pre-existing values edited in the course.
To import your .CSV file into your Gradebook in Canvas, first navigate to your course site's Grades page. From here, you can click on the Import button in the upper right hand corner of the page.
This will take you to the Upload Gradebook screen. Click Browse... to find the desired .csv file on your device. Once you have selected that file, select the Upload Data button in order to begin the import process.