Here's a rough skeleton for the click code for a control button that sets a particular field (MyField) to a particular value ("NewValue") for all the visible records on a form, as the form is currently filtered.
Assuming that your current code is VBA, that code would be placed in the lines between .Edit and .Update, and it would need to be modified to make it use the rst recordset, rather than using the current record. (If that makes any sense to you.)
Code:
Private Sub cmdSetAll_Click()
Dim rst As Recordset
On Error GoTo Err_cmdSetAll_Click
Set rst = Me.Recordset
With rst
.MoveFirst
Do
.Edit
!MyField = "NewValue"
.Update
.Movenext
Loop Until .EOF
End With
Exit_cmdSetAll_Click:
Exit Sub
Err_cmdSetAll_Click:
MsgBox Err.Description
Resume Exit_cmdSetAll_Click
End Sub