This should be enough to UPDATE the udf table:


UPDATE udf
SET Value = CASE udf.fieldno WHEN 1 THEN udf1 WHEN 2 THEN udf2 WHEN 3 THEN udf3
  WHEN 4 THEN udf4 WHEN 5 THEN udf5 END
FROM udf
INNER JOIN UserImport UI ON UI.userid = udf.userid

Full example: 

use tempdb;
create table users
(
userid int,
firstname varchar(25),
lastname varchar(25),
middlename varchar(25)
);

insert into users values (1,'some','dude','big');

create table udf(userid int,fieldno int, value varchar(25));

insert into udf values (1,1,'123456');
insert into udf values (1,2,'text1');
insert into udf values (1,3,'01/01/2018');
insert into udf values (1,4,'Text2');
insert into udf values (1,5,'Text3');

create table userimport
(
userid int
,firstname varchar(25)
,lastname varchar(25)
,middlename varchar(25)
,udf1 varchar(25)
,udf2 varchar(25)
,udf3 varchar(25)
,udf4 varchar(25)
,udf5 varchar(25)
);

insert into userimport values(1,'some','dude','big','New1','New2','New3','New4','New5');

SELECT 'Before', * FROM udf

UPDATE udf
SET Value = CASE udf.fieldno WHEN 1 THEN udf1 WHEN 2 THEN udf2 WHEN 3 THEN udf3
  WHEN 4 THEN udf4 WHEN 5 THEN udf5 END
FROM udf
INNER JOIN UserImport UI ON UI.userid = udf.userid

SELECT 'After', * FROM udf

drop table users
drop table udf
drop table userimport