You cannot explicitly select which sideband a spectral window will be placed in, since the OT automatically configures a solution based on the defined spectral windows. However, if the currently defined spectral windows are all in one of the two sidebands, you may be able to change the sideband in which these are placed by appropriately setting the other (e.g. continuum) spectral windows. This may be advisable in the case where the image spectral window in Bands 9 or 10 (shown on the other side of the LO1 frequency in the spectral visualiser) falls in a region of bad atmospheric transmission, negatively impacting the noise level in the original spectral window.
Note that while you can move around the LO1/sideband display in the spectral visualiser, this is for experimentation purposes only and does not change the spectral setup.