If you got the correct permission, you can edit a page or blog post. In 'view mode', to change the content in a page, click Edit or press E on the keyboard and you will enter 'edit mode'. Now you see the page in 'edit mode' and you can make your changes. Click Preview to see what your changes will look like, or save your page immediately. You will then see your page again in 'view mode'.
You can see how this works in Video #8 of our Video Tutorial series "Learn Atlassian Confluence".
You may also check out this page in Atlassian's documentation.