How would I reinstall a operating system on a laptop?

search 'how to restore to factory settings ....' and put your brand of laptop and version of windows.

if your having issues loading into windows, put 'from bootup' on the end of the search

you will find instructions on how to wipe and reinstall the operating system, drivers and any programs that came with the laptop. this will delete all files on the laptop.

most laptops have a restore partition that can be accessed by pressing an F button after switching on , only use the repair option or it will delete all your files and data

Download windows installation to a USB stick. Install from there.

Specifically- "which laptop". And what OS?

Help us answer you.

Edit: and while you are at it...would you consider Linux- or do you "need" windows?

Different methods for different systems