First, log in to your [Sakura Mobile My Page]. 
LINK: https://myaccount.sakuramobile.jp/auth/login

After logging into your "My Sakura Mobile" account, please go to the "Manage Plan" page
For information on how to get to your "Manage Plan" page, visit here.

1) Once on your "Manage Plan" page, please select "Change Plan" (Refer to ① in the images below)

2) Please select your desired plan (Refer to ② in the images below)

3) Once you have confirmed the above information is correct, please select "Request change plan" to submit your request (Refer to ③ in the images above)