You can use any web browser on PC or Mac. If you are not logged into your YouTube account automatically, click the blue Sign In button located in the top right of the YouTube webpage.  Then select the Google account associated with your YouTube account. If none of the accounts listed are associated with your YouTube account, click Use another account and sign in with the email address and password associated with your YouTube account. This is where your profile picture normally goes.  If you do not have a profile picture set, it will be a colored circle with your initial in the middle.  This displays your account menu. It's towards the bottom of your account menu.  It's next to an icon that resembles a gear. It's the blue text next to your name and profile image at the top of the Settings menu.  This opens the "About Me" page for your Google account. The circular icon in the middle of the page contains your current profile image, or a colored circle with your initial.  Click the white camera icon in the middle to display the "Pick a Photo" window. It's the first square in the upper-left corner of the "Pick a Photo" window.  This displays a file browser you can use to select a photo. Use the file browser to navigate to a photo file on your computer.  Different folders are listed in the sidebar to the left of the file browser.  Click an image file to select it, and then click Open in the lower-right corner of the file browser.  Make sure you have a file saved to your computer that you can upload. Alternatively, if you have photos already uploaded to your Google account, you can click one of those photos in the "Pick a photo" window. It's in the upper-right corner of the "Pick a photo" window.  This confirms the changes you have made to your account.  The photo you select applies to all your Google accounts, including your YouTube account.

TL;DR in Vietnamese:
Truy cập https://www.youtube.com bằng trình duyệt web. Đăng nhập tài khoản YouTube. Nhấp vào biểu tượng ở góc trên bên phải. Nhấp vào Settings (Cài đặt) nằm cuối trình đơn tài khoản. Nhấp vào Edit on Google (Chỉnh sửa trên Google). Nhấp vào biểu tượng nằm trong hình tròn. Nhấp vào Upload Photo (Tải ảnh lên). Chọn ảnh và nhấp vào Open (Mở). Nhấp vào Done (Xong) ở góc trên bên phải cửa sổ "Pick a photo".