Not quite sure what you meant, so I'll give you two difference answers.
If you are talking about creating a new, empty file, just delete (or rename) and then create (ie touch) a new file with write permissions to Informix and root.
If you are talking about changing it's location (ie. directory) that's a function of the onconfig variable BAR_ACT_LOG. I'm not sure when this parameter is read (ie once at startup or whenever needed) so you may need to bounce Informix to get the new path to take effect.
To change the location of the log file ,change the location in the
$INFORMIXDIR/etc/$ONCONFIG : BAR_ACT_LOG parameter. You don't have to bounce the server, this takes effect when the Onbar command is executed.