It is not clear how the sentence you provide is reported speech. Do you mean to change it so that it appears to be reported speech being quoted by a third party? If so, you are probably going to need to change 1st person to 3rd person.
He said he learned, if he faced another question about designing a structure, how he could use the formula.
(I've used masculine pronouns, but it could be feminine).
As mentioned in the comments, the 'that' in the original sentence should be removed. The sentence as it stands is not grammatical.
It would probably be written best as:
He said he learned how to use the formula if he faced another question about designing a structure.
Reported speech can be in the first person as well. In which case, you just need to add the words that make it clear you are dealing with reported speech: 'I said' at the start of the original sentence.