At any time you may purchase a tier for for 150 Vbucks in Fortnite Battle Royale.
You can do so by going to the Battle Pass screen and clicking on the "Purchase Tier" button on the bottom, then confirming.
Doing so advances instantly the level of your Battle Pass, and you immediately unlock all rewards of that tier.
- When you purchase a tier you preserve your tier progression in terms of Battle Stars.
- So for example if you were at tier 26 with 4/10 Battle Stars to the next tier, you will now be at tier 27 with 4/10 Battle Stars to the next tier.
- So essentially you are buying 10 Battle Stars.