You don't. You can only downgrade without having CFW while are on ≤11.3. If you are past that, you have no available method of installing CFW that's free. If you have a flashcart, it may be ntr-boot compatible, so look into that. If you don't, look into SeedMiner (Single System DSiWare method). For SeedMiner, you will need to buy a DSiWare game that's compatible with the method (suggested one is a Sudoku for 1.99 as it's the cheapest).