There's a syntax error in insert and update query.
your insert & update statements should be changed as below,
Insert Statement -
"INSERT INTO Tabela(Oznaka, ReN, RmN) " & _
" VALUES('" & Me.txtOznaka.Text & "' ,'" & Me.txtReN.Text & "','" & _
Me.txtRmN.Text & "'')"
Update Statement -
'update data in table
cmd.CommandText = "UPDATE Tabela " & _
" SET Oznaka='" & Me.txtOznaka.Text & "'" & _
", ReN='" & Me.txtReN.Text & "'" & _
", RmN='" & Me.txtRmN.Text & "'" & _
" WHERE Oznaka=" & Me.txtOznaka.Tag
Try out with this code..
Hope this will work for u..