[HELP] how to change color space standard correctly when upscale footage?

[Selur]

so when i try to upscale 720×480 to 1920×1280, is it need to set like this?

Yes, and 'Rec.709' would have been automatically selected it you resize from SD to HD.

what's the pre and post clipping mean?

What is unclear about the tool-tip?

Clipping:
Specifies whether pre/post clipping with limiter to 16-235/16-240 should be used.

Possible settings:
- no clipping
- pre clipping (clip input to ColorMatrix)
- post clipping (clip output from ColorMatrix)
- pre and post clipping (clip input and output from ColorMatrix)

Default: pre and post clipping