Once assets are added to XUMM exchanging them is as simple as selecting the currency then the Exchange button.
There is a video of the process here. Don't blink or you'll miss it.
In fact it is so simple and quick for the safety of the users it was decided to only allow trades at a reasonable rate of exchange.
If the spread and/or slippage is too high, XUMM doesn't allow in-app easy exchange protecting users from getting a bad rate.
Spread is the difference between the prices quoted or an immediate sale and an immediate purchase
Slippage refers to situations in which a market participant receives a different trade execution price than intended.
Slippage occurs when the bid/ask spread (the price) changes between the time a market order is requested and the time an exchange or other market maker executes the order.
When XUMM sees 4% or greater spread OR 4% slippage it shows the following message in red on the Exchange screen;
The price difference between selling and buying offers is too high
Users can always use a 3rd party tool to create offers. When that message is shown XUMM offers a link to the XRP Toolkit site.