How do I support my exochorda macrantha

You don't say how big it is, but if the rootball's above the ground, it needs to be dug up, carefully, retaining as much of the root ball as possible, and replanted in a deeper, larger hole, fed and watered. It's keeling over because it's not been planted properly, or there's some obstruction under the ground where it's planted that either cannot be removed, or the previous owners couldn't be bothered to remove.

Thank you so much. The roots are looking very woody, will that affect the re-planting?

Wow what a quick response.

It may have been rocked by the wind over winter. The plant waves about and digs a hole around itself. If it is in a windy site you could put in a sturdy support, but keep it low down to avoid it looking ugly. but you need to do as Bamboo suggests or it will dry out badly over summer.

I've tied my exochorda macrantha to a trellis. It works well, and the branches arch over with pretty white flowers.
Fixing some of the branches to trellis can be done in an attractive way and might help the shrub from rocking about. I hope this helps.

Thank you all so much, I really appreciate the advice.

Good luck with your shrub.
I hope it has lots of flowers next spring.
Mine took a few years to really get going.... :o)

Such a shame that it flowers for such a short time, only about 2 weeks.