Change a slug

I hope someone can help with this issue as it’s driving me nuts and I’ve spent all afternoon trying different plugins – none of them work and hoping someone can give me a solution.

I’m building a website where I want to use the portfolio as a different thing called ‘activities’, however I cannot get the slug off – it shows http://www.mysite.com/portfolio/item I want to either get rid of the word portfolio completely and change it to ‘activities’. I changed the word portfolio in the theme folder by replacing it but nothing happened 🙁

Thanks