Spreadsheet program for iOS and macOS - the iOS version
This apple support page shows a list of current shortcuts available in the iOS version of numbers.
This is surprisingly non-obvious. You double-tap it to select the text, then you can edit it from there. I thought there would be something involving a long-press, but that only allows you to move the sheet around. Similarly, a single-tap brings up an edit menu that allows you to cut (remove and place in pasteboard), copy (place in pasteboard), duplicate, or delete the sheet.