SQL Query syntax to CASE

I have code that updates a table, but when moving the data the size is larger than what our field allows.
I need to cast the field.

This is my current code, which works if the data was only 20 characters long.

UPDATE a
SET a.I797 = b.Description
from bluedot_eimmigration.dbo.Users a
INNER JOIN [bluedot_eimmigration].[dbo].[zzz_eimmigration_customdocuments] AS b ON b.userid = a.userid 
WHERE b.DocName = 'I-797'

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I am trying to make the description 20 characters so that I can update without getting an error

UPDATE a
SET a.I797 = CAST(b.Description AS VARCHAR(20) ) AS Description , 
from bluedot_eimmigration.dbo.Users a
INNER JOIN [bluedot_eimmigration].[dbo].[zzz_eimmigration_customdocuments] AS b ON b.userid = a.userid 
WHERE b.DocName = 'I-797'

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But the above gives me a syntax error:

Msg 156, Level 15, State 1, Line 15
Incorrect syntax near the keyword 'AS'.

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Your help is appreciated.