Hello! I made a Post-it Note based on a video by GIMPtricks. I really like it but I do have one question though: somehow in the process of making the image the font got changed from Marker SD to something else. How do I go back and change both the font type as well as the font size? As always, any guidance is greatly appreciated.

If the text layer isnt rasterized yet (there is a T in the layer name), then you can just click the text layer and click the text tool icon in your tool box. Now just click the text on the image window itself and using the font selector re-select your desired font.You can use the other options to change size and what not.

If the font is rasterized -the T is no longer there.The text is no longer editable through the text tool.
You will probably have to create another text layer and make the original text layer invisible or simply delete it.

Thank you. I don't see a "T" in front of the text layer so it must be rasterized. How did I do that? I should probably know so I don't accidentally do it again.

Actually anytime you do anything other than whats allowed in options it will probably raster it.The best way to tell is to look and see if the T is there.