Restore Thumbnail Backup Photos to Original Size

I have found dozens of wanted thumbnail size photos (PNG files) on my old backup hard drive. The files have not yet been extracted. My question is how can I restore these thumbnail PNG files to original size?

Thank you

Not been extracted from what? You can resize them but they may well be very grainy depending on what size they were. Generally if it's a thumbnail enlarging it will make it unusable

Samuria said:

Not been extracted from what? You can resize them but they may well be very grainy depending on what size they were. Generally if it's a thumbnail enlarging it will make it unusable

Old backup files found on my old external hard drive.