You have to remove the headlamp bezel for access to the ring retaining screws. The ring should have three mounting tabs for the screws.
If you are using aftermarket headlamp rings, they generally are made from thin gauge plated steel, are relatively flimsy and will rust in the first 12 months of use. The GM #5964577 headlamp ring is made from stout stainless steel for 7" round headlamps. If you need new rings, the original factory part is expensive, but still available and worthwhile.
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