convert command, something like:
convert input.jpg -quality nn output.jpg
You can also resize them to something smaller (here, 2000px in the margest dimension) with a little bit of sharpening to compensate the slight blurring introduced by the scaling down (unless the pictures are already heavilty processed (smartphone pictures):
convert input.jpg -geometry 2000 -sharpen 0x1.0 -quality 85 output.jpg
convert produces a new file (so you can use it as an evolved
cp command). Otherwise from the same package the
mogrify command does everything in place.
As an amateur photographer: just don't. Getting a low quality version of a high quality picture is easy. The revers is nearly impossible. What is 1TB nowadays? I'm not even sure it will take less time to recompress and backup tan to back them up directly (unless you back up to the cloud).