You can do as
update
calendar
set DateTo = concat(
substring_index(DateTo,'T',1),'T','13:00'
)
where substring_index(DateTo,'T',1) = '2014-02-19'
;
As MatBailie suggested instead of
where substring_index(DateTo,'T',1) = '2014-02-19'
its better to use
WHERE DateTo LIKE '2014-02-19T%'
This will be significantly faster if there is an index on DateTo