You're never locked into a position on Predictions on the Betr app. You can exit at any point before a market resolves, and there are two ways to do it.
The first is a market order, where you sell immediately at whatever the best available bid is in the order book right now. The second is a limit order, where you name your own price and how many shares you want to sell, then wait for a buyer to come to you.
Keep in mind that a limit order only goes through once someone else is willing to buy at the exact price you've set. Until that happens, your shares stay right where they are.