You may have recently noticed that a slider switch at the top-left of your Microsoft Teams desktop client appeared in Windows. It asks if you'd like to "Try the new Teams". As almost all of the tech we use for our daily work is online, or "in the cloud", all of us should get used to the fact that our applications and web services will change from time to time. Sometimes, screens will change, buttons and controls are moved to a new location or renamed, or new features are added. It just means we all need to be more flexible and ready to adopt such changes.
In the case of the new Teams change, virtually all are "under-the-hood". The client was rebuilt from scratch and now it loads and runs faster and uses much less of your device's memory. You will see some screen differences in your channels and meetings, but on the whole the new client is recommended. Switching to the new Teams is optional and you can switch back if you really want to. Microsoft will concentrate development and feature updates on the new client, so at some point, all users will be required to switch to it. It's better to make the switch sooner, rather than later.