'Turn on' is a separable phrasal verb, which means that the full form of the direct object - 'the computer' - can be put between 'turn' and 'on', but a pronoun ( 'it' ) can't. So you can say: Turn on the computer Turn the computer on Turn it on But you can't say 'Turn on it'.

but if you use the pronoun "it" you can only use "turn it on"

i think they both work