MySQL 8.0

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CREATE TABLE gallery(
image_name varchar(50) );

insert into gallery values
('16240154071618319441c1.png'),
('16240154png618319441c1.png');

Records: 2  Duplicates: 0  Warnings: 0

select * 
from gallery;

image_name

16240154071618319441c1.png

16240154png618319441c1.png

UPDATE `gallery` 
SET `image_name` = replace(image_name,(right(image_name,4)),'.webp');

Rows matched: 2  Changed: 2  Warnings: 0

select * 
from gallery;

image_name

16240154071618319441c1.webp

16240154png618319441c1.webp