Yes, you can post links. It does help many times to see what is going on, but in this case it sounds like the templates in the Control Panel are going to be the focus. (And, of course, we do not encourage links to the control panel!) Since it sounds like no instructions were left with this site, your best bet is to open up the template for the main web page.
There is a chance that the slideshow was hard-coded, and if that is the case then it may be buried in a Global Variable and referenced from the main template.
If the slideshow is not hard-coded, you should find the code surrounding the slideshow code (JavaScript or HTML) with {exp:webblog:entries} tags. That tag would tell you which channel, and possibly which entry to look for to change the slides.
These are the most certain ways to find it. If the slides have captions, you may be able to search the entries for that caption also.
Hope this helps. Let me know how it goes. 😊
EDIT: This does assume that no external slideshow builder was used. I have seen that happen before.