I repotted a camellia yesterday but used multi purpose compost by mistake.
That compost is on the bottom, but I used rotted manure around the root ball.
What should I do?
Well, if you only did it yesterday, it's not a problem - what could be a major problem is surrounding the rootball with manure. Go and get some ericaceous compost if you don't have any. Turn the Camellia out, remove the manure, empty the pot completely. If you really must use manure in a pot, make sure it's well rotted and smells just earthy (personally, I don't recommend the use of rotted manure in pots, unless you know 100% how it's been produced/composted) then either mix a small amount in with your ericaceous compost, or put a layer on the bottom of the pot, fill with ericaceous compost, then repot the Camellia into that.
Were it my Camellia, I'd just stick with ericaceous compost and provide a separate fertiliser for use with acid loving plants in subsequent years in spring.