Hi JC2023. I am an Excel user like you.
I don't know if there is something unusual going on with your situation, but here is what should be the case.
The Data Validation icon is in the Data Tools section of the Data ribbon. Depending on the resolution of your computer's monitor these icons are sometimes labeled, sometimes just an icon like below, and sometimes you have to click on a dropdown for the particular section to see the items in that section (if this is the case look for a dropdown for Data Tools). This is the icon for Data Validation:
Select one or all of the cells that have Data Validation applied and then open the Data Validation dialogue.
The Allow line will tell what kind of data is allowed. The Source line will tell you where it is found. The checkbox for In-cell dropdown is how you indicate that you want to see the dropdown arrow in each cell when it is selected. At times data may also be directly entered in the Source line rather than being taken from a range somewhere else. Once you find the source of the data you should be able to edit the data in the list's location and edit the range in the validation window.
If you do edit the range in the validation window, be sure and checkmark the box at the bottom of the window to Apply these changes to all other cells with the same settings.
If your Data Validation is not working correctly, you might consider clicking on Clear All and re-doing the Data Validation from scratch. Make note of the source first so that you can find it again to set up the new validation. I will also usually save a copy of the file I am working on with a slightly different name (add a 1 or something to the end of the name) so that if something doesn't work right, I can go back to my original file without losing anything.
Reply if you have additional questions or information. Please mark this reply as answered if this solves your question.
Rich~M