[Q] shrink varchar2 size and truncate data????

we have a table has a field define as "varchar2(2000)", we tried to change size from varchar2(2000) TO VARCHAR2(1000).  dUE TO SOME RECORDS DATA LARGE THAN 1000. 
 

I can use following sql statemnet to find which records  > 1000 characters, but how can I truncate data larger than 1000 characters?
 

select id, length(INSTRUCTIONS) from functions where length(INSTRUCTIONS) >=1000;

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