Change site from sub to root domain?

Not even sure if this is the right terminology.
My site is prospanish.co.uk/wordpress_a and I want to change it to just prospanish.co.uk

I’ve tried copying the files/folders to the root and changing the address in the blog, but it doesn’t work. I didn’t want to install beacuse I thought it would wipe what’s already set up in the mysql.
Any ideas?
Thanks