To reimport, first select the data you want to overwrite, then redo the import step.
For example here is a simple dataset with a graph using the data. Start by clicking any row of the data to select.
Next, you want to select all the data. One quick way is to use Command-A.
Then, use Data > Import Excel Sheet, as you did before. Select the data and click Import Selection.
Then the data is overwritten and all the graphs using that data are automatically updated.
If you open your excel sheet, you can also copy and paste from Excel into DataGraph. That would also require you to first select the data in DataGraph so you paste over the values.
Does this help?