No, once a username is finalized, it can’t be changed in any way, including its capitalization.
The one exception is if you created a Reddit account with your Google account or Apple ID as your login, and haven’t finalized your assigned username yet. Learn how to change your username if you’ve signed up with your Google account or Apple ID.
Also, keep in mind that if you delete your account, your username will not become available again and will be unusable. If you’d like to create a new account with a new username, you’re more than welcome to create a new one and keep both your accounts. (In fact, you can create as many accounts as you’d like.)
If you have more questions that aren’t covered in these FAQs, check out r/help. Someone may have had the same question, or you can ask a new one and a Reddit administrator or one of your fellow redditors will help you out.