All of the text in the DotNetNuke core is localized, as well as all of Engage Software's commercial modules, and many other 3rd party modules. Therefore the best way to alter the text, if you have access to a SuperUser (Host) Account, is to use the Language Editor. From the Admin menu, choose the Language option. In the drop down action menu in the upper left hand corner of that control, choose Language Editor.
Once there, you can use the tree view on the left to navigate to the resource file of your choosing. Many of the DotNetNuke core resources are listed by default. Specific modules will be in the Local Resources folder. Once you get to the module you'll typically find the resources in an App_LocalResources folder. After you select the correct resource file, you can see the various messages defined in it, which you can change right there. Once you've made your changes, you can click the Update link at the bottom of the page to make the change permanent.
It's all as easy as that. Customize your DotNetNuke installation to your heart's content.