According to the Home Guide at San Francisco Gate, a ceiling fan blade can be replaced using a ladder or a set of steps and a screwdriver, along with the new blade. The old blade needs to be unscrewed and removed prior to the new one being set in place.
Having the correct replacement blade for the brand in use is essential; if one by the same manufacturer is unavailable, purchase a compatible blade from another manufacturer.
Before attempting to change the blade on a ceiling fan, the power to the fan needs to be turned off, preferably at the breaker switch. Set up the steps or ladder so that the fan can be reached comfortably without having to lean or over-stretch. Hold the blade that needs to be replaced, and carefully undo the screws that hold it in place. Remove the blade, put it aside and be sure to keep track of the screws. Hold the new blade in place. Check that it is in the correct position and oriented the same way as the other blades. Screw the new blade in place with the old screws, and ensure that they are tight. It is advisable to check the other blades fixtures at the same time to make sure there are no loose screws. This is also a great opportunity to dust the other fan blades if they require it.
When finished, return power to the fan from the breaker, and switch it on. If there is any wobble in the new blade, switch the power back off, and tighten the screws a little more until the blade no longer shifts during its rotations.