How to alter the data type of a column on DB2

set data type

See the example below:

Table Name:  fruit 
Column name: color

 db2 describe table fruit

Output:

Column                         Type      Type
name                           schema    name               Length   Scale Nulls
------------------------------ --------- ------------------ -------- ----- ------
COLOR                       SYSIBM    DECIMAL                  10     0 No  

Altering the data type

db2 alter table fruit alter column color set data type decimal(20)

Checking the results

db2 describe table fruit

Output:

Column                         Type      Type
name                           schema    name               Length   Scale Nulls
------------------------------ --------- ------------------ -------- ----- ------
COLOR                       SYSIBM    DECIMAL                  20     0 N

WARNING:

Don't forget to reorganize your table. The "Reorg recommended" column identifies the data type alterations that will require table reorganization before a table can again be fully accessed (SQLSTATE 57016)

db2 reorg table fruit

db2 runstats on table fruit and indexes all

If you will alter the varchar(n) column to varchar(n +x), 
it is the only situation that you don't need to reorg your table.