I believe it might be first challenge to update text of TextInput because it's a little bit far from intuition steps. Most easiest way to understand how to build up this is to test for yourself. Here is an one of simplest articles to describe the steps.
Simple steps how to update text on TextInput
Open https://create.powerapps.com/studio/ and choose "Blacnk App" by following an image below.
Choose "Insert > Text -> Text Input" to put TextInput control on your app.
Choose "Insert > Button" to put Button Control on your app.
You can find two controls on Screen1 like below.
You need to utilize variable to update the text. Click your InputText control, choose "Advanced" and update Data - Default as variable name - ex. "val01" like below. You can avoid error at this time because this error will be fixed after Button control setup.
Click your button control, choose "Advance" and update "OnSelect" formula like below.
Reset(TextInput1);
UpdateContext({val01 : "hello"});
Debug to update text on TextInput
Execute this test apps and click button, and you can confirm behavior below.
