OK, the Express is the least expensive model of Roku. Honestly I never recommend it because it has occasional issues like this. For not much more cost, Roku offers much better players. Right now, the 4K Streaming Stick is only $25. You might not care about 4K, but it's a far more capable performer than the basic Express.
Now, to your issue. The next question will be: does it only reboot itself when using the same channel, or is it happening with multiple channels? Have you attempted to move the Roku player to a location that might have better airflow (in case it's overheating, although it usually pops a message on screen when that happens)?
If it's only happening with a single channel, the next step would be to remove the channel, reboot the Roku (critical step, don't skip) then add the channel back in again. If it requires a login, you'll have to do that again, but this would remove any corrupt code with that channel and install a clean version.
If it's happening with all channels, and moving it to a different location made no change, then it's probably time to do a factory reset. This removes everything but the base operating system and restores it to new, fresh out of the box. You then have to set it up again from scratch, which I acknowledge is a pain. But when you attach it to your Roku user account all channels you have installed now will automatically be added back. You just have to enter any user credentials again. Yeah, a pain.
Don't do a factory reset unless nothing else works. It's a last gasp attempt to repair, and quite often does the job. Of course, if there's something wrong with the hardware, none of this will resolve anything and a new player is the best option.
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For late model Roku Stick: Remove the battery cover from your remote control. Press the button just below the batteries for several seconds. Your remote will trigger a software update and perhaps some queries to answer on your tv screen. Good luck.