The present specification proposes a signal processing method for changing an operating mode for transmitting a PPDU in a wireless LAN system. Specifically, a first station receives, from the second station, indication information which indicates a change in an operating mode indicating a receiving channel bandwidth and a number of spatial streams supported by a second station during a TXOP interval. When the number of spatial streams and the receiving channel bandwidth decrease, the first station configures a PPDU for the second station by using the indication information after transmitting an ACK for the indication information. When the number of spatial streams and the receiving channel bandwidth increase, the first station configures a PPDU for the second station by using the indication information during a subsequent TXOP interval of the TXOP interval.