On your menu screen, you’ll see the Bumble Boost and Boost Premium options underneath your profile photo. Click on the option you would like to subscribe to. 

Alternatively, you can tap on the pixelated square in your match queue to view your Beeline. There you will see the option to subscribe to Boost Premium. 

Finally, you can click on one of the silver circles you see around the photos of your expired connections. These are the expired matches that you missed!