In error I used an installation cd of my old Dell laptop which had XP, so ended up downgrading OS on my Inspiron N5010 from Windows 7 to XP
How can I get back to Windows 7 ?
The Inspiron laptop did not come with a installation CD of windows 7
Unfortunately Windows 7 OEM is not downloadable.
Your best *** is to ensure you have the latest BIOS revision for your model. A15:
Then download the Windows 10 TH2 Insider preview .iso and use it to clean install Windows 10 on your system using your Windows 7 OEM key:
Windows FAQs and OEM Downloads