@Lippy Thanks for reaching out
You can navigate to the overview page of your service bus namespace and click on Basic option under the pricing tier. A new window will pop up and you can select the standard pricing tier and click on Select.
For your reference highlighted the action on the below screenshot.
Let me know if you need any assistance.
Please click on 'Yes' if it helped so that it can help others in the community looking for help on similar topics.