SQL Server 2017

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create table t1 (id int, trn int);
insert into t1 values(999,null),(1000,200),(1101,null),(1003,null)
,(1008,44),(1002,1)
6 rows affected

update t 
 set trn=rn from
  (select trn,row_number()
   over (order by id) rn
   from t1
   where id>=1000 and id<=2000) t
5 rows affected

select * from t1 order by id


id
trn


999
null

1000
1

1002
2

1003
3

1008
4

1101
5