StashApp has full support for the Stellar DEX, meaning you can access it all through StashApp.
To swap funds directly within StashApp click on Exchange, as in the image below:
The top assets is the asset you want to sell; select the asset you want to sell and enter the desired amount. The bottom asset is the what you will receive; select the asset you wish to buy. If you fill in an amount for either the top or bottom asset, the other will automatically be filled in with the estimated of the number of tokens you will receive/sell (based on the current exchange rate and available liquidity). Finally tap on Swap to complete the transaction.