It's my birthday today (Thank you for the gift by the way!), But I was wondering, how do I use the 10 wonderful rerolls given to me?

Once you select a horse you'd like to reroll, in the many links for managing the horse will be the option to "reroll horse" . Pick that! :)

Thank you!

What does a reroll do exactly?
(Sorry, Ment to ask this in the first place)

Rerolls change the pattern or sometimes vary the base coat color. All patterns and base coats have multiple variations so when rerolled, you'll get different variations. If that makes sense? ^^

Say you have a Tobiano with a belly band. If you reroll, you might get Tobiano instead with shoulder and butt splotches.

Thanks again!

This makes a lot more sense now uvu