You have an option to use either button or dropdown list as your collection picker.
This procedure is applicable in both themes 2.0 and 3.0.
STEP 1. Under Sales Channels, select Online Store.
STEP 2. On the right portion of the page, click on Customize.
STEP 3. On the top menu, select Collection pages from the drop-down list.
STEP 4. On the left panel of the page, under Collection pages, tap Collection pages
STEP 5. Under Collection pages, tap tag picker type drop-down list and select your preferred tag picker.
STEP 6. Click on Save to save changes.