Does the XP increase when you keep retakung the test or should I reset?

Answer 1

If you reset a course you lose all points for that language including the ones you got from challenges.

Answer 2

Jennie Matela from what I experimented, if you retake the course lessons, you only get 1 xp.
I wouldn't advice you to reset your course. Doing so will result in you loosing all the xp you got from that course.

Answer 3

You can do challenges to increase xp IIRC. Or by unlocking different badges.