MySQL 5.7

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create table mytable(mycolumn varchar(100));

insert into mytable(mycolumn) values('\\\\uploads\\\\photos\\\\')
SELECT REPLACE(mycolumn, '\\\\', '\\/') 
FROM mytable
WHERE mycolumn LIKE '%photos%';

REPLACE(mycolumn, '\\', '\/')

\/uploads\/photos\/

UPDATE mytable
SET mycolumn = REPLACE(mycolumn, '\\\\', '\\/')
WHERE mycolumn LIKE '%photos%';
SELECT * FROM mytable

mycolumn

\/uploads\/photos\/