I have a lilac tree in our small front garden which has grown far too high
for the space. How can I prune it so that it flowers at a height of about
5

best time to prune is just after the bush/tree has flowered. it might be
wise to get it to the shape you want over a couple of seasons rather than
all at once ... although you don't say how high it is at present. you could
prune it now, or into the autumn - but you will be cutting off next year's
flowers in the process.