Context for this one is everyone is alive, what season 8? Nope, never happened, what do you mean. Established Adam/Shiro.
After the war things are difficult for a while. At some point Shiro knew he'd have to give a full recap of events to Adam, trying his best to dodge around the mention of his death, the clone or anything about the astral plane.
It takes him a while to process it all, rightfully so, his fiance is still alive after all.
During this time period they still hang out together, Adam sharing bits and pieces of his life whilst Shiro was gone, giving context to the dodgy timeline of events Keith told him once.
But as more time goes on, they fall back into a familiar routine. It starts small with hand holding, cheek kisses etc. At some point date night returns, then later on they move back in together.
And even though Shiro feels like Adam would still want to keep their former fiance title, he one day decides to take their old rings and get them adjusted.
Looking at them reminds him of how young and naive they were when they got together. Your typical highschool sweethearts.
He remembers how nervous he was when he bought Adams ring, stressing out majorly over if it was enough, was it too soon (as if they weren't together for years and basically already married outside of legally.) He couldn't afford anything too fancy back then, but he needed Adam to know that he was committed. And luckily, he said yes. The symbolism being far more important than the cost.
Of course, he didn't know that Adam had kept his ring on the entire time, some part of him hoping Shiro would return.
This time, he decides that he wants the ring to be a bit more showy, add something sparkly or colourful that really makes it stand out. And so he spends a really long time asking about options and trying to decide what they can do. Eventually he finds the perfect option.
Cue Shiro being just as nervous as younger him when he proposed the first time. Adam knows somethings up but can't pinpoint what until the moment finally happens and he hears the question. "Will you still marry me?" And that charming smile Shiro does sweeps him off his feet once again.
When they finally do get married, Adam can't wait for all the day he gets to tell people about his husband who wanted to marry him twice.