You can open dictionary of 'Description' field and change the type as 'html'. PFB SS.
However, if you are trying to do this for any table(for example incident or any custom table), which is extended from any table (for example task table) and if description comes from parent table only, then you can't change the field type of description even by overriding dictionary. There is no way.
It is recommended that you can create a new field with label 'Description' (name, for example, u_description) with your desired field type and make the original 'Description' field hidden.
There is another challenge you may face that if have a newly created custom field 'Description' (name, for example, u_description) already have some data already in it then you may not be able to change the 'field type' since it will result in loss of data stored in that custom 'Description' (name, for example, u_description) field.
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Thanks,
Animesh Das
