We are sorry to hear for your leaving and quit as a UNIQLO account member.
For UNIQLO website user, please login your online account and refer to below steps:
1) Click [Human] icon on top banner and select [Profile].
2) Choose [Withdrawal from app membership] under [Profile settings].
3) Click the button [WITHDRAWAL] after reading the withdrawal agreement.
For UNIQLO App user, please refer to below steps:
1) Open the UNIQLO App.
2) Choose [MEMBERSHIP] on bottom banner.
3) Choose [PROFILE SETTINGS].
4) Choose [WITHDRAWAL FROM APP MEMBERSHIP] and tick the agreement.