2015ChevyExpress2500

Hey there guys. I just bought a used commercially owned 2015 Chevy Express 2500 4.8 V8 w/ 104,000 miles. It runs really strong, has a very clean body, so far so good(fingers crossed). The Carfax states it was taken to Stevens Creek Chevrolet in San Jose, Ca for service. I went to the dealership for intel. In fact 5 months ago the engine seized and the previous owner had the entire engine replaced for a little under $8,000. So it basically has a new(probably used) engine. The only problems I noticed was a very light buzzing noise coming from the 12 volt plug on the dashboard in the start of acceleration...

My first plan of attack was to change the oil, I did last weekend, and I put Mobil 1 Advanced Full Synthetic and a nice Mobil 1 filter. My goal this weekend is to change(but not flush) the vans coolant. I have the new updated 1996-2019 Haynes Repair Manual and of course the directions are very general. I haven't checked if there's a drain valve on the left side of my radiator yet. Any advice/instructions would be greatly appreciated. Also, is it ok to put the pre-mixed auto store DEX-COOL? I'm just doing this fluid change for good measure, after this I will change the transmission oil/filter... Thank you very much, any advice is greatly appreciated!