FlowJo allows you to edit the keyword values of your FCS data (only in the current workspace, the actual FCS data does not get modified).

First, add your desired keyword into the workspace (this is best done once the FCS data is in the workspace): click workspace menu, Edit columns. Choose keyword(s) from the list on the left and add them in.

Now that your keyword is in the workspace, click on any of the cells that are in parallel with the FCS file (not one of the nodes belonging to the file) and type over the existing value.

First, add the keywords you wish to alter. In this example, the $p#n keywords were added.

Now, click the cell you wish to edit. Notice the value that is selected changes its background to white.

Type over the value to change it. Press enter to finalize the editing.