Snake on my Casio Graph 25 + calculator

No, I don't have a USB port but some kind of jack plug, I don't know.
And is there a tutorial because there are some buttons I don't know.
Thank you in advance.

Hello,
In that case, you can't copy the program from a computer. If you don't want to rewrite all the code by hand, you can always copy it from a friend's calculator who has this game (that's what the jack port is for).
And what tutorial are you looking for? If it's to know how to use it, you can find the manual in PDF here:
http://www.casio-education.be/calculatrice_casio_documents/notices/Graph25+_Fr.pdf

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I'm looking for information on how to play Snake on my calculator in video format because I'm not sure if I need to switch to alpha mode and type in letters or if there are specific keys to use.
Thanks in advance.

Indeed, you should not use letters as they only serve as variables or to display text. The functions you need can be found in the different menus. This tutorial to learn programming may guide you on where to find the various functions:
https://www.planet-casio.com/Fr/programmation
Otherwise, the functions are often quite well organized, so you can find them by searching through the different menus.

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I forgot to mention that I'm not good at programming calculators; I would like a video because otherwise it's too complicated.