Ok,
I've the name of the columns updated so I want to compare the old value and the new value using deleted and inserted tables...

I put the following test statement into the update trigger:

...
set @sql= 'select * from deleted'
exec sp_executesql @sql
...

But everytime I update the table the trigger returns

Server: Msg 208, Level 16, State 1, Line 1
Invalid object name 'deleted'.

How can I reference deleted and inserted tables to know which values where changed?