Open the conversation with your exchange partner in the app and tap on "Details" at the top of the screen.
Once you've tapped "Details", you can check all the details of your exchange: exchange type, dates, GuestPoints, number of travelers, etc...
To change the exchange type, simply tap on the current exchange type "Reciprocal" or "With Guestpoints" and a window will open where you will be able to choose again between the two types.
When you see an arrow next to an exchange detail, it means that you can edit that detail. If there is no arrow, only your exchange partner can edit it.
Once all details are updated, you will have the option to pre-approve the exchange or your exchange partner will have to pre-approve it if you made the exchange request.
Once the exchange is pre-approved, the last step is to finalize it by clicking on "Finalize" at the bottom of the screen: