Can anyone suggest how I can change the length of an existing column (For ex. column name -- ABCD, column_Type -- Text, Length -- 254). It would be great to have solution on platforms like ArcGIS or QGIS.
In QGIS it is worth to say that it is not possible to change field width (if we are talking about shapefiles) for existing columns. But it is really easy to create new column with needed length and copy content into that column from the old one. You can do that with Field calculator. For renaming / reordering / adding multiple fields etc. there is great plugin called table manager - https://plugins.qgis.org/plugins/tablemanager/.
If you don't need to change the column length but only shorten the string you can use Field calculator again. For example for column named Title first 10 characters:
And this question is almost dupplicate of - How to change field length in QGIS - you can alter column length in dbf files (the tabular data connected to shapefile) in OpenOffice Calc - as advised in comment below this has some strict limitations and can lead to integrity/file corruption.
Also as advised by other comment if you need to go over limit - maximum length (for example text 254 (256)) specified for shapefile (respectively linked dBase file) data type, you need to use different format (like SpatiaLite, PostGIS etc.), there is no other way around: