To edit a WordPress page or post, you first need to login to the dashboard.
You do this by going to the url:www.example.com/wp-admin
with your website location
You will be prompted for your username and password which you should either have been given by your site administrator or entered when you set up the site.
On successful login, you will be at the WordPress Dashboard.
You will notice links on the lefthand side for, amongst other things, Pages and Posts. Select the menu item you require (Pages are usually static content, Posts are usually blog entries or news articles) and you will see a list of All Pages or All Posts.
Hover over the post you wish to edit with your mouse and four links will appear: Edit, Quick Edit, Trash and View. Select 'Edit'.
You can now make changes to the Page or Post content on your site.
When you have finished editing your Page or Post, hit Update near the top right.