hello,what is the size of tread for diamondback insight 1 pedals,I would like to upgrade the pedals ,is it 9/16? thanks

Unlike a lot of things that screw into a bicycle, pedals are not something you have to worry about. All pedals have the same threading. All. I forget the thread pitch but you use a 15mm open ended wrench to install them. And the pedals on the left side of the bicycle ... you spin the wrench clockwise to remove (loosen)! And counterclockwise to tighten! This is vitally important to remember.

Not wholly correct as far as all pedals having the same threading but typically these days most adult bikes are 9/16" and many kids bikes are 1/2" but there were other standards and back in the day probably most of which have since bitten the dust. Some pedals also use a 6 or 8 allen key in the back to tighten rather than a flat that would use a 15mm pedal wrench.

Also anything with metal on metal contact use grease!!!!!!! If it is titanium use anti-seize/copper paste. Seized pedals (as well as seatposts) are no fun for anyone to deal with.

I was going to put a line in my earlier post about how someone was going to bring up the exceptions but I thought that would be mean ...

There's certainly no harm in mentioning that there are exceptions, especially if you explain why they probably don't apply to the Diamondback.

Keep in mind that if driving from the back side you will need to reverse the rotation vs using a wrench on the front.
I use Tef-Gel on my pedal (and all other dissimilar metal) threads and also add stainless pedal washers which help stop seizing and also reduce the marring of the cranks which can occur. https://www.tefgel.com/contain.php?param=tefgel_infor

Two sizes, 9/16" and 1/2", will cover just about every bike made in the past few decades.

Bikes with two- or three-piece cranksets, including your Diamondback Insight, will use pedals with a 9/16" spindle (and 20 threads per inch). Just about every 'nice' road bike, mountain bike, and hybrid will use this pedal size.

Bikes with a one-piece steel crank -- typically kids' bikes and cruisers -- will generally have 1/2" pedal spindles. The crank arm is narrower since the whole crank needs to be threaded through the frame's bottom bracket shell to install it. Narrower crank arm, narrower pedal hole.

Bike aint French , (so no mention of that 3rd exemption to the norm).

Correct!

Never heard of Tefgel? Looks like it is a boating type supply so it seems like it would do the task quite well. I might look into it further just for gits and shiggles.

Krytox 240 AB Grease would be my top reccomendation when I don't mind spending a little extra on grease when I am wanting to save a little I go with the Krytox LVP Grease.