If you have not looked or don't know, GM like the other OEM's use a chrome cover over the lug nut. They look good but are just a PITA over time. As you can see the stock ones already have dents and the car only has 5k miles on it. The tires were rotated by the dealer. I will be willing to bet that you will have a problem with one or more lug nuts if you rotate your tires or remove them for cleaning especially if a impact gun is used.
NOTE -The stock lug nuts use a 19mm socket and the torque spec is 110 psi.
Should you decide to DIY them you will need a jack (if plan to rotate your tires), a torque wrench and a set of lug nut sockets. I have all of these at my shop, but it is 50 miles from the house. So I made a trip to Harbor Freight and picked up what I needed. All of it was on sale. The 1.5 ton aluminum racing jacket was on sale for $59 but they were out of stock so the manager gave me the larger version, still 1.5 ton but bigger size wise, for the sale price. The torque wrench was on coupon sale for $12 and the lug nut socket set was on coupon sale for $10.
Next you will need some lug nuts..... There are any number of suppliers and the price can run as high $50 for 20. I picked the Gorilla brand and got them online for .74 a piece in bulk pack.
They come in various sizes and colors. But, Cadillac uses 12mmx1.50 Acorn Bulge style with a 60° cone and are about 1.40" tall. Bulk part numbers are as follows:
17MM Chrome - 31138
13/16 Chrome - 91138
13/16 Chrome Heat Treat - 91138HT
13/16 Black Chrome - 91138BC
3/4 Chrome Heat Treat - 41138HT
3/4 Black Chrome - 41138BC
3/4 Red Chrome - 41138RD
3/4 Blue Chrome - 41138BL
I wanted Red so I went with the 3/4" version. I checked the replacements with a calipers and they matched up to the stock ones.