How to change the template of an Impress presentation?

I’ve got a presentation that was done in Powerpoint using a blank generic template and I’d like to bring it into LibreOffice Impress and do some changes, including changing the template (slide design) to match my other presentations. But everything I’ve tried results in either a blank new document with my preferred template, or the old pptx with the wrong appearance. I may be missing something but would appreciate any suggestions to make this happen, thanks. Bob