Update meta_value for specific word

all links that starts with https://test.com/ need to be changed to https://www.test.com/

In an update you can do:

update t
    set meta_value = concat('https://www.test.com/', substring(meta_value, 17))
    where meta_value like 'https://test.com/%';

Note: You might want some condition on the meta_key as well.

Assuming // occurs only once in the string (reasonable for a URL), you could simplify this to:

update t
    set meta_value = replace(meta_value, '//', '//www.')
    where meta_value like 'https://test.com/%';