Yes. Open the gear in the player's control bar and pick a rung under Quality. Every course video is encoded at three of them, 1080p, 720p and 360p, plus Auto, which is where you start until you choose otherwise.
The three rungs are the same on every lesson
Course video is converted into a fixed ladder of 1080p, 720p and 360p, each carrying the same audio. That is deliberate, so there is always a low rung to fall back on when a connection is poor, and the choice does not change from one lesson to the next.
A rung is a size, not a promise of detail. A lesson filmed at 720p and played at the 1080p rung does not gain anything that was never recorded, and it costs more data.
Auto reads your connection and the size of the player
Auto is the default and it is the right setting most of the time. It has one behaviour worth knowing: it will not pull the 1080p rung into a small player. In the standard page-width player, Auto can settle at 720p and stay there however fast your connection is.
To get the top rung, either switch the player to Cinema mode or Fullscreen, which gives Auto a reason to climb, or pick 1080p by hand. A hand-picked rung overrides both the connection test and the player size.
The Auto entry shows what it settled on, written as "Auto (720p)". That is the quickest way to see what you are actually watching.
The choice lasts for that lesson only
Moving to the next lesson resets the player to Auto. Your selection is not stored against your account and does not follow you to another device.
Where the control is, and when it is missing
The gear sits with the other controls along the bottom of the player. It holds playback speed, from 0.5× to 2×, with Quality below it. The Quality section is only drawn when the player found more than one rung to offer.
On a browser without Media Source support, which includes some older iPhones and iPads, playback falls back to the device's own video handling and no quality menu appears. The device chooses for you there.
If the picture keeps softening and sharpening, that is Auto stepping up and down a weak connection, and fixing it to 360p by hand stops the switching. A picture that stops entirely rather than softening is something else: see course video stops playing after a few hours.