As you know, you can access Teams using either Teams client for Windows, MacOS, mobile or web browser.
As you may also know, a new Teams client has been made available and now is generally available.
Well, the new Teams client is becoming available also on web browser too.
The rolling out has already started, if it is not yet available for you just try again in few days.
To start using the new Teams web client (when available to you), you will just have to turn on the switch Try the new Teams showing up top left and confirm the switch