How to prune my Paeony???

Now, is this a herbaceous paeony or a tree paeony? If the former then all you do is remove the dead stems in autumn or spring and let it grow.
If the latter then really speaking you should not prune it at all and eventually it will flower at the correct height.

Owdboggy - stupid question but how will I tell which one it is ?

it is best not to prune a peony they are i bit fussy

a tree peony has woody stems like a woody shrub and in the autumn bare wood/twigs/branches. the herbaceaous one all the leaves die off and in the spring reddish swollen buds like lipsticks push up through the soil. hope that makes sense and helps you identify it.

Ah, I see - mine is a herbaceaous one then - thanks for that

all you need to do is remove the dead flower head and then cut the dead leaves back to ground level late autumn.