Hi,
if the column Description(50 Char) doesn't have any value (or values can be overwritten) and you need to copy values to it from Longdescription(200 char) column, you can use the next query:
update Mytable set Description=substring (Longdescription, 1, 50)
if the column Description(50 Char) has some values that you don't want to be overwritten, you can add values from Longdescription(200 char) to the end of the Description(50 Char) column:
insert into Mytable (Description)
select substring(Longdescription, 1, 50) from Mytable