How can I upgrade to a higher level?
If you are talking about language level, you have already done that. If you are looking for a way to get to a higher level quicker, you can do the placement test at the beginning of learning a language or you can scroll down your learning tree to test out of 6 or more skills.
By earning xp. This is done by learning new lessons, reviewing old ones, and working on translations in the Immersion tab.
I've completed all the lessons at the basic levels, but when I take the test to pass higher, there are lots of words that have not been introduced yet at the lower level. I feel stuck. What should I do?
You don't need to take a test to progress. You simply need to continue through the skills, as they unlock. (If they have failed to unlock, make sure you've done all the lessons in all the skills on the line you are currently on.)
The progress quiz has nothing to do with your ability to continue through the tree. It tests you on the entire content of the tree, thereby giving you a baseline against which to measure the results of future tests.
OK. I've complete Alphabet, Basics 1, Phrases 1 , and done the Strengthen Skills ad nauseum! What do I do next to progress? Basics 2 remains locked. That's where I want to go.
Make sure you've done all the lessons in all the skills on the line you are currently on.
Have the icons (the round circles on the Home page) not unlocked? If they have, they should be some color other than gray.