The left speaker will be on your right-hand side, since the laptop is laying upside down.

Slide the point of a spudger under the left speaker connector and gently lift it straight up and out of the socket.

Don't lift on the cables themselves, or you may damage them.

During reassembly, hold the connector over its socket and press down gently with the flat end of a spudger until it clicks into place. If you're having trouble, don't try to force the connector into its socket—reposition it and try again.