Tap to exit is probably sufficient, but there are many options and only one way to know for sure.
Most preview modes are non-interactive, so tap to exit works pretty well. It may not be the most intuitive, but unless the user suddenly becomes completely passive at that point they will touch the screen eventually and end the preview.
However, if your use case involves scrolling the preview around a lot, it could be too easy to dismiss by accident. In small doses this is probably not a substantial risk (if users don't preview things that often, or they don't have to scroll that far each time) but if the preview is a mile long and they really want to see what's at the bottom, a double-tap or long tap might be another viable option since it's less likely to occur by accident.
None of these options is really that discoverable, and if a user's attention wanders and they forget that they are looking at a preview and not the real thing you might want a message or a button informing them of preview mode and how to exit. The upside here is that it also serves as a great interaction focus for ending the preview when they are ready. If you don't want to cover up part of the preview, you can integrate it into the content that is being shown in the elements they customized.
User testing will get you more definitive answers.
We can speculate all day and offer other interactions, but the only way to be sure is to have real people try it in the real world. Something that seems good on paper could be confusing or worse when someone actually tries it. You might be over thinking the problem and everyone taps to exit without even thinking about it.
Because this is a narrow interaction, it probably doesn't need to be the focus of its own test. You could create a test where the last task is to try the preview mode and then ask them to exit it. Watch what they do. If most people try tapping or swiping or start looking for an exit button, there is your answer.
If there isn't a consensus, or if you want to do something different from what users intuitively wanted, you'll need to explain how to exit in the UI and can refer to some of the suggestions above.