For those who don't know, this was added to the Global Server as a _limited_ song back in September 2020, and unfortunately that promotion has ended long ago. I do wish they'd bring it back though!
You can upgrade them thru the EX Missions. You assemble a team from the same band (e.g. all Afterglow), and then you try to get increasing Free Live scores (get 800k, get 900k, ...) At certain scores, you unlock more spaces in the band frame. It's best to just read your currently available EX Missions for more details ✌
all the event stamps will eventually make their way into the michelle sticker exchange, but the seasonal (new year, summer, halloween, christmas) stamps can only be earned as bonus for rolling a limited gacha last year.
Log-in everyday, try to get T1k in a lot of events, and don't roll too much gacha ^^ this is still nothing though compared to people who actually buy stars XD