So one of my player as an idea for a character that i want him to play, cause its simply a cool character Idea, but its not something playable in the base game. The Idea is a demon that has been banished from hell basically and lost most of his powers on the way, so the only spell he has should be Charm Person i think, cause it is fitting for https://1921681254.mx/ a demon (and he agrees), but i have been trying for an 1 hour now to make an homebrew race based on the tiefling's race bonuses on DnD Beyond, but i cant manage to just change the spell from Themathaurgy to Charm Person, is there a way ? Am i dumb?

If you are having probolems switching thaumaturgy for charm person as a homebrew on d&db then it might beecause Thaumaturgy is a cantrip and Charm Person is a level 1 spell.

However if you have access to Mordenkainens Tome of Foes you can use the Fierna subtype of Teifling which grants Friends in place of Thaumaturgy, Charm person in place of Hellish Rebuke and Suggestion in place of Darkness which migh tgive you the feel you are after.

You can see a walkthrough on how to properly attach known spells to a race/species in the Homebrew Subclasses & Species FAQ #14: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). With those instructions you can attach any spells to which you have access to your exiled demon race.