'Learn' is one of several verbs which have two possible past tense/past participle forms, one regular and one irregular. 'Spell' and 'burn' are others. The irregular form tends to be used more in British English and the regular one in American English, but it really doesn't matter which you use. Both are correct.
The only time it does matter is if you are using the past participle as an adjective. For example, you say 'a piece of burnt toast', but not usually 'a piece of burned toast.'. There is an adjective 'learned' (pronounced as two syllables) to describe someone who has studied a great deal, for example 'a very learned man'. This is a situation where 'learnt' can't be used.