The leap to the C1 level is no small feat. If you really want to get there, there are important habits you must consider. Just coming to an English class is not going to be enough. Having a B2 level is quite good already, though. You might choose to remain there (I hope you don't).
• Communication depends upon mutual intelligibility. That is to say that it is only possible if the language forms produced by the speaker are identified and understood by the listener. It is, therefore, the responsibility of speakers to pronounce them as intelligibly as possible (pronunciation and stress patterns).
• A considerable enlargement of concrete vocabulary.
• The refinement of functional and general notional categories.
• Recognition and limited control of important register varieties avoiding excessive formality on the one hand and colloquial or familiar usage on the other.
• Increased ability to understand and produce longer and more complex utterances.
• Interaction is less constrained and both partners can act in a more flexible and natural way, following basic goal-directed conversation strategies rather than adhering as closely as possible to fixed verbal exchange patterns.
• Greater sociocultural and sociolinguistic competence. The everyday use of any language is impregnated with the culture of the community that uses it to organise its communication.
• Improved reading skills applied to a wider range of texts.