The pronoun 'it' in the sentence leads to an ambiguous reference because it's not clear whether 'it' refers to the act of learning or to computers.
The problem with pronoun reference in the sentence, 'Over the summer Frank learned a lot about installing and repairing computers. In fact, it changed his career path,' is ambiguous reference.
The pronoun 'it' could refer to the act of learning about computers or to the computers themselves, and the sentence does not make clear which antecedent 'it' refers to.
To clarify the pronoun reference, one could revise the sentence to directly state what changed Frank's career path, such as 'this experience' or 'this knowledge.'