Both spelt and spelled are two different spellings of the past tense of the verb 'spell'. The spelling tends to vary based on the version of English you're using: In some versions of English, 'spelled' is the preferred variant, in other versions English, 'spelt' is is the preferred variant.

https://www.grammarly.com/blog/spelled-spelt/

Both spelt and spelled can be used as the past tense and past participle forms of the verb spell. They have the same meaning and are used interchangeably.

You might use spelled but that does not mean that spelt is wrong.

Most regular verbs take -d or -ed endings in the past tense (climbed, rushed, smoked, touched, washed) while some have retained their older
-t endings (built, felt, lent, meant, spent). But a few troublemakers have alternative -ed and -t endings –

burned, burnt
dreamed, dreamt
kneeled, knelt
leaped, leapt
leaned, leant
learned, learnt
smelled, smelt
spelled, spelt
spilled, spilt
spoiled, spoilt

Spelt is also a type of grain but words can have more than one meaning