Lower radiator hose

The hose is plugged in and held in position by a locking clip. Use a screw driver to pry the clip outwards until it is unseated from position and clicks into its unlocked position. The clip should stay on the hose end. Twist the hose until it is loose and it should come off. Inside the hose is an 'O' ring. Lubricate it with a little rubber grease before installing. For new hose, do not pull the clip out. Just plug into the radiator and thermostat outlet until you hear the clip clicks. Tug at hose to ensure that is is locked, failing which the hose would detach itself when pressure builds up as coolant heats up.

Accessing from top is possible but the space is quite cramped. If there is anything that is preventing easy access, they should be removed.
The hose should come with the clip in its locked position. If it is not there, you would need to reuse the old one. The clip comes easily when pried off.

Normally such jobs are rather easy to remove and replace, less than one hour but after replacing, you would need a little time to monitor if there are any leaks.