how do i fix shower switch that has a fishy smell coming from it?

Get an electrician in to change the switch.

From a previous Fishy Smell problem...

"Aliphatic amines have a fishy smell and are liberated when plastic electrical fittings get over-warm and particularly as they age.

Typically, 13-amp plugs are in fact unable properly to handle 13 amp loads (i.e.3Kw) and will always get warm if handling such loads. Sooner or later they will start to smell as will plastic lamp holders fitted with high-wattage bulbs.

Alan Sutton (C.Chem., FRSC)"

http://www.howtomendit.com/answers.php?id=4106

Search on here for Fishy Smell -

Peccavi
February 2013