I want to download a standard CustAccountStatementEXT. Cause the one I work with now, is a custom and it doesn't show the customer payments.
I'm assuming you are talking about the CustAccountStatementEXT report. To return to the uncustomized version of the report, you can simply delete the customizations in the development layer (but make sure you export a copy of the customizations in case you want to go back to the customized version). Please be aware that the report consists of several AOT objects, so to go back to the uncustomized Version, it may not be enough to just delete the report modifications.
This may also be a good time to review the Dynamics AX layer concept.