You can change the slicer styles in the Pivot Table whenever you want to change, please see below for details, or (see please check here to insert a slicer to Pivot Table):
Step 1: Click anywhere on the slicer;
Step 2: Click the "Slicer" Tab in the ribbon if you work with the Excel 365;
- The "Analyze" Tab if Excel 2019 or the "Options" Tab if Excel 2010;
Step 3: Select one of the slicer styles.