If then in sql string

I am trying to write an sql statement and I know Im wrong below.

Essentially, if tblRegistration.Overage = 3, then AttendanceGrid Set X = 'X'
and  if tblRegistration.Overage is not 3, then AttendanceGrid Set X = ' '

How do I write that correctly below

Update AttendanceGrid Set X = 'X'
FROM tblRegistration INNER JOIN AttendanceGrid  on tblRegistration.id=AttendanceGrid.id 
and tblRegistration.Overage = 3
Else
Update AttendanceGrid Set X = ''
FROM tblRegistration INNER JOIN AttendanceGrid  on tblRegistration.id=AttendanceGrid.id 
and tblRegistration.Overage < 3

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