all answers are great but I just want to give you a good example
select replace('this value from table', 'table', 'table but updated')
this SQL statement will replace the existence of the word "table"
(second parameter) inside the given statement(first parameter) with the third parameter
the initial value is this value from table
but after executing replace function it will be this value from table but updated
and here is a real example
UPDATE publication
SET doi = replace(doi, '10.7440/perifrasis', '10.25025/perifrasis')
WHERE doi like '10.7440/perifrasis%'
for example if we have this value
10.7440/perifrasis.2010.1.issue-1
it will become
10.25025/perifrasis.2010.1.issue-1
hope this gives you better visualization