You can use a program by FunOrange called "Osu! Trainer" but it wont be ranked and in osu! Lazer there is a difficulty adjustment mod, but I dont think thats ranked. McOsu has this as well, but again its unranked.
- McOsu is great for that. It's available on Steam and you can increase or decrease the AR, HP, OD or CS at your will. You'll no longer play on Stable but there will be more options for you.
- Osu! Trainer is a program available on GitHub useful to change the approach rate & more for you while remaining on Stable. If you want to be as quick as possible, I advise you this.