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In the course, use the control panel to access the full Gradebook.
On the upper right there are a series of icons, right below the Student Preview button. Select the Download buttons, which looks like a box with an arrow pointing downwards.
On the sidebar that comes up, you will be able to download either the full grade center or a specific column. You can also select to download the Grade History, which provides the history of changes made to individual grades. You will also be able to select the file type of the download; select a .csv file for the file type, as there are problems importing the tab delimited format into newer versions of Excel. When you have chosen all your settings, click 'Download' to download the file to your computer.
When you need to upload this file, follow a similar process. This time, select the upload button, which looks like a box with an arrow pointing upwards. 
Using the sidebar that pops up, you can either browse for the desired file, or drag and drop it into the space. Select Upload to upload the file onto your Gradebook.
