Hi burger_moon! There are a couple of options:
1. Attempt to sell back to the company
2. Sell your shares to an independent buyer that you've found (the company typically has right of first refusal, which just means they're allowed to outbid the independent buyer)
3. Wait until the IPO, and sell them on the open market.
4. Sell your shares on EquityZen, SharesPost, or something similar.