You can make an offer on a property in the app from the property screen by tapping the Offer button at the bottom of the screen.
When making an offer, provide your offer amount and select the radio button for whether you are purchasing the property with cash or a loan. Note that in general financial institutions will not provide conventional loans on distressed properties. If you're interested in using a conventional loan, consider messaging the seller prior to making your offer.
You can change your offer as long as it's not accepted by tapping on the menu and selecting Buying, then selecting the property that you made an offer on.