Erase my MacBook Air 2017

estefania248 Said:

"Erase my MacBook Air 2017: How do I erase my MacBook"

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Reinstalling the macOS anew:

You'd backup your Mac, and then install the previous macOS. A backup isn't required, but it is advised you create one prior to this so that you can have something to restore your data from, should something go wring afterwards.

1. Create: a Time Machine Backup using an external Hard Drive
2. Install: from a CD created for macOS Mountain Lion.
3. Migrate: items from your TMB
4. Download then Install: the applications, getting the programs back from the Developers.

This Apple support article has some information on the subject.

Use Disk Utility to erase an Intel-based Mac - Apple Support

estefania248 Said:

"Erase my MacBook Air 2017: How do I erase my MacBook"

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What's your Device?

I see 10.8 is on the device pertaining to your post. MacBook Air 2017 cannot have macOS Mountain lion installed on it.