It is normal that we use a presentation more than one time for different conference. Anytime doing that, modifying the footer, where the conference’s name and the date are, is an important job to do to avoid embarrassing moment.
It is pretty obvious that you can simply open Slide Master and edit directly the footer text to whatever you want. However, for Powerpoit 2016 and later, when you do that and return to the slide, the old text content is still there and the modification you’ve done is applicable only for new slide.
In order to edit that, now what we should do is manually edit them. Not slide by slide but by Insert -> Header & Footer -> Modify the content, then click “Apply to All”.
It’s a nuisance but we can solve it by that trick.