How to append some letters or numbers to already existing data
You can use the CONCAT() function:

select concat(255, 720967537), concat('my ', 'test')


Results:

255720967537, my test


SQL Fiddle with Demo

If you want to UPDATE the values in your table, you could do the following:

update t
set col1 = concat(255, col1)
  , col2 = concat('my ', col2);


See SQL Fiddle with Demo