Hi,
If you haven't done one before then it will be easier to have some help
ie, two sets of ladders one either side, spare pair of hands really useful.
Start at swan neck and try smashing with a lump hammer or grind sections away, be warned the cast iron is very heavy espeically when your up ladders !(making sure nothing falls in side.)
I have replaced lots this way, usually on my own which beleive me is heavy work and a little dangerous (needs must) make sure no one will be walking around underneath Ie small children. Take your time and be very careful wielding the grinder around. Once you get midway it becomes much easier. Source a suitabe drain connection if your not replacing the whole rest bend. Oh small pieces of iron DO fly out and can kickback the grinder ALL SAFETY no worries😎 Good luck😀