If your signal builder block contains data from an existing CSV file, you can replace it with data from a new CSV file with these steps:
1) Open the signal builder block and click File > Import from file. Browse to the new CSV you would like to import.
2) In the "Data to Import" window, select all.
3) In the "Placement for Selected Data" menu, choose "Replace existing dataset". Then, click "Confirm Selection". Click "OK".
4) In the "Import File" window that pops up, select "No, import without saving".
I am not sure what release you are using, but these steps work for me in the current release, MATLAB R2017b.