Arms is getting a beta (though Nintendo is calling it a Testpunch) and the first opportunity to play will be next week.
Much like Splatoon 2’s beta (A.K.A. testfire), it will occur on specific dates. Those dates are May 26, May 27, May 28, June 2, June 3, and June 4.
You can download the necessary software on your Switch to be ready on those dates immediately.