There are products you can buy from garden centres and I think there's an old fashioned remedy of putting iron nails in the soil but it's a bit hit and miss. I've come to accept with some disappointment that my lovely blue hydrangea is now pretty much all pink.
Bethbilly your soil needs to be more acidic. You can use an ericaceous compost or treat in with iron and aluminium sulphate. Growing Success Hydrangea Colourant works quite well, but you need to treat the soil through the autumn and winter for best results.
Unless the plant is in a pot with the correct soil (see below) it will eventually change colour. I've tried all the things mentioned and even then my beautiful blue hydrangea turned to a sort of mauve colour :-(((
Too late for this year but get some sequestrene and water your pink hydrangeas in the spring. I did this only once, many years ago and mine has remained bright blue ever since.