A Microsoft file hosting and synchronization service.
Hello,
Microsoft personal account and business account belong to two different/separate ecosystems, so you cannot move your business account to the personal side of products.
However, you can definitely move the files.
The easiest way to do so is - do you have both OneDrive personal and business accounts signed in via OneDrive desktop apps? If so, you should have two folders already in your File Explorer. Just move files directly out of the business folder into the personal folder. Make sure you move it in batches as it is challegning to move the entire 1TB or huge size of files in one batch.
If you have subscriptions in the business account, go to Admin Center https://admin.microsoft.com > Billing > Your products and cancel them.
Then you need to sign up for products on the personal side if you need them.