You can do this a few ways. The easiest will likely be the Data tab -> Text to Columns. Select the column and split the data by the comma. Then you can use a simple concatenate to bring them back together in the other order.

Another method would be to use the text functions as follows (assuming the data is in call A1, and your formula is in cell b1):

=RIGHT(A2,LEN(A2)-FIND(",",A2))&","&LEFT(A2,FIND(",",A2)-1)