The following one line should do it:
xori $t0, $s0, 1
Explained: the contents in $s0 contains zeros and ones, while the immediate value has zeros and a one in the LSB. Whenever the LSB is 0, it is xored with 1 and outputs a 1. Whenever it is 1, it is xored with 1 and outputs a 0. The remaining bits will output a 1 if they are 1 and a 0 if they are 0 because they are being xored with 0, thus preserving their state.