How do I use an UPDATE statement in a function?

Any suggestions?
I'm getting an error when trying to use an UPDATE statement inside of a function.
error: Invalid use of side-effecting or time-dependent operator in 'UPDATE' within a function.

I am getting the same error as well.
Here are the code for the same.

I would like to convert one procedure to function and call that function into select statement.

Procedure listed as below:

CREATE PROCEDURE Test_NextEmployee

AS

SET NOCOUNT ON

DECLARE @iReturn int

BEGIN TRANSACTION

SELECT @iReturn = Sequence FROM .dbo.EMPSEQUENCE (TABLOCKX) -- set exclusive table lock

UPDATE dbo.EMPSEQUENCE SET Sequence = ( Sequence + 1 )

COMMIT TRANSACTION

SELECT @iReturn

RETURN @iReturn

And after that i have to call that function into below Insert Statement in place of AutoKey column.

Insert Into EmpSequence (KeySegment, Keystn, KeySegmentType, FinlSegment, Status)

select 'Auto key', eop.Keystn, 0, a.FinlSegment, 2

from KeyEOP KP

join KeyEOPSegmentDetail a on a.KeyEOP=kP.KeyEOP and a.updop < 2

join Kfinl f on eop.keyeop = f.keyfeop

join FinlSegmentDetail ff on f.keyfinl = ff.keyfinl and ff.updop < 2

and ff.finlsegment = a.finlsegment

join stn I on i.Keystn=EOP.Keystn and Class in ( 8,15)

where EOP.UpdOp<>2

Can anyone please help with that?

Thanks