The present specification suggests a signal processing method for changing an operating mode to configure a PPDU in a wireless local area network (WLAN) system. Specifically, a first station receives indication information indicating a change in operating mode that indicates a number of spatial streams and a receiving channel bandwidth that are supported by a second station from the second station. The first station transmits a block acknowledgement (BA) in response to the indication information, the BA including a first delay required bit. The first station defers applying the indication information so that the indication information is not applied when the first delay required bit is a predetermined value.