People have linked some cheap app things, which may work, but the real deal stuff is something like a Trinnov rack unit, or Neumanns DSP room correction system, which is what I use. Both are somewhat pricey, but this stuff isn’t audiophile junk, it’s meant for studios that really need the room to get out of the way, and it’s usually paired with quite a bit of treatment. The difference is insane. It’s not like a “this one has a tiny bit more sparkle” difference, it’s a +-15db at a couple of points in the low end kind of difference.

Someone also linked RoomEQWizard, which is an awesome tool, but dialling in a flat sound with it using just an EQ is going to send you to phase hell, and you’ll also need a halfway decent measurement microphone to get the most of it.