Where <TableName> is the name of the table you want to update, <Column...> are the names of the columns (fields) you want to update and <IdentityColumn> is the name of a column that allows to uniquely identify a row (record) in the table and <KeyValue> is the value for this column. The values could be derived from controls on a form:
... FirtsName = '" & Me.FirstName.Value & "'"...
You could also use a Recordset:
Dim strSQL as String
Dim rst as DAO.Recordset
strSQL = " SELECT * FROM <TableName> " & _
"WHERE <IdentityColumn> = <KeyValue>;"
Set rst = CurrentDb.OpenRecordset(strSQL, DbOpenDynaset, DbSeeChanges)
If .EOF = False Then
!<Column1> = <Value1>
!<ColumnX> = <ValueX>
Set rst = Nothing