To change the cafe style, you need to buy as many items in the new style as possible and get rid of the old-style items, sell them or put them in the warehouse. The style will be changed when the new style's tips are the highest in your cafe.

After that, wait until the offers in the bank are refreshed, and you will see the devices in a new style among the offers, which you can purchase to replace the old ones.

Keep buying furniture and equipment in your new style, and Fernando will give you a clue about the current level of tips if it's needed.