Before you change the profile pic of your client or your business, be sure you have the right logo or picture that you need to put as a profile picture.
Open Youtube

Click on YouTube studio

This will open the YouTube Studio page

Click on your channel

Click on the camera icon on your profile picture

This will open the settings of your account. Click on the change button next to the picture section. Your profile picture needs to be 98 x 98 pixels and 4MB (or less)

That's it! You changed your profile picture successfully!