Convert English(US) spelling to British spelling?

I have a book written in US English for the American market.

I wish to convert it to British spelling and then sell this book in the UK too.

I have reset the computer spelling in Apple Mac Book Pro preferences.

I then reset the preferences in IBook Inspector, as suggested in a previous thread for a new iBook file using Inspector/Text/More/Language

I then copied and pasted my chapters across.

The text still is in US English...

I am wondering what is the best way ahead....

Have I missed out a vital step or is there another step to do for each page?

Ken