In DNN (DotNetNuke), if you want to change built-in form field's label text and their associated tooltips, then try to find relevant fields info in the associated App_LocalReources folder. Here all static text info of the relevant fields is placed which you can modify.
For example, to change registration module's forms field's tooltip (which appears by clicking on the little question marks to the left of each field) data - just browse to admin\Users\App_LocalResources folder of your DNN file structure and change User.ascx.resx file.
Hope this helps!
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dnn, tooltip