I don't know that you can, can you ?
I know that they 'hate' root disturbance so i'm going to wait and see what answers come through, Bamboo's around - she'll know.

If you absolutely have to, then when it's dormant - and as Louise says, it may upset the plant. Cut it back, and dig the planting hole so that it's prepared with compost - bigger than the rootball you intend to dig out.

Then dig all round the plant and take out the plant - with soil, too - because if you damage the roots, bye-bye Clematis, in all likelihood. Put it on a sack and drag it to its new home. Plant immediately, deeply, and water well. You'll have to water it for its first year as you would do with a new one. Feed it in the spring, and keep your fingers crossed.