If you have access to multiple boards within LENS, you can use the "Board Picker" to switch in and out of the various boards.
click on the Boards icon on the left navigation bar
This will show you all the boards you are a member of. Click on the tile of the board you'd like to open.
Note: if you do not see the "Boards" icon on the left navigation bar, this means that you do not have access to multiple boards. If you'd like to learn how to browse the boards that you are not a member of and request access, check out this article.