Open YouTube Studio
Click Content
Select the video
Click Details
Go to Thumbnail
Choose an auto-generated thumbnail or upload a custom thumbnail
If uploading, use an image with a 16:9 aspect ratio
Make sure the image is clear and high quality
Add large, readable text if needed
Use bright colors and a strong focal point
Save the changes
If on mobile, open the YouTube app
Tap your profile picture
Go to Your videos
Select the video
Tap Edit
Tap the thumbnail area
Choose or upload a thumbnail
Save the video details