Installing a toilet in an upstairs bathroom in a home or residence doesn't really differ from adding a toilet into any other room of any other floor of the house, but attention must be paid to avoid leaks, which will certainly damage rooms underneath the upstairs bathroom. For those with a little know-how, this DIY-type job can often be completed in an afternoon.
Turn off the water supply that feeds the bathroom. If you're replacing an old toilet, use an adjustable spanner to remove the holding bolts on the base of the existing toilet, remove water from the bowl using a bucket, and pull up on the toilet to remove it from the floor. Slide the toilet out of the way and dispose of it properly. Peel away the old sealant from the flange, reapply new sealant, bolt the new toilet in place and turn the water back on. Move onto the next step if you're installing a new toilet.
Measure and saw a hole for the flange -- which generally speaking measures 10 cm (4 inches) in diameter -- in the floor. It is assumed lead-in piping is already in place underneath the floor that should've been installed when the bathroom was built. If not, it's recommended you consult a plumber to avoid costly mistakes.
Slide the flange into the hole in the floor and hook up the end of the flange to the existing piping. Generously apply sealant around the joint between the floorboard and the flange to both secure it in place as well as to waterproof the joint to prevent premature erosion, which would result in leaks.
Slide the gasket over the exposed top of the flange; this provides a gateway between the toilet and the flange. Again, generously apply sealant to the joint between the flange and the gasket to prevent leakage. Move the new toilet in place, over the gasket head, and bolt the toilet in place with new galvanised bolts that are rust resistant.
Turn the water supply leading to the room back on and allow the tank to fill. Flush the toilet once and check for leaks along the base; if present, repeat the steps to apply more sealant to all related components.