Your Missions can be accessed under the Missions button in the lower left corner of the home screen or by pausing the game during a run.
Here you will see your current Mission Set containing 3 Missions. Completing a full set will add an extra point to your Score Multiplier.
The Score Multiplier is currently capped at x30, so after your 30th Mission Set, you will instead be rewarded with Super Mystery Boxes!
If a Mission is causing you trouble, you can always skip it by using coins. . . or sometimes by watching an ad.
You can get an additional Score Multiplier on top of your x30 by using a Bonus Character – which changes every Season Hunt!