You will have to locate the evaporator coils inside the fridge. In a side by side, they are probably in the bottom. You will have to remove some plastic parts to get to the coils. Under the coils is a drip tray which catches the water when the fridge goes into the defrost cycle. In the drip tray, there is a small drain hole; about 1/4 inch diameter or less. The drain hole can be difficult to find, but trust me it's there. Use a turkey baster with HOT water to melt the accumulated ice inside the drain tube. You can also use a hair drier; just be careful not to damage any plastic parts.
Hope this helps.