Trying to use VBA to perform an Automated find and replace function

Ishmale 

Please give this a try and let me know if this works as desired.

Remember to tweak the worksheets and ranges if not as per the sample data you provided.

Sub ReplaceOldUserCodeWithNewUserCode()
Dim wsData      As Worksheet
Dim wsUser      As Worksheet
Dim x           As Variant
Dim dict        As Object
Dim Rng         As Range
Dim Cel         As Range
Dim lr          As Long
Dim i           As Long

Application.ScreenUpdating = False

Set wsData = Worksheets("Daily Pick Data")
Set wsUser = Worksheets("Click USER Ref")

lr = wsData.Cells(Rows.Count, 1).End(xlUp).Row
Set Rng = wsData.Range("A2:A" & lr)

x = wsUser.Range("A1").CurrentRegion.Value
Set dict = CreateObject("Scripting.Dictionary")

For i = 2 To UBound(x, 1)
    dict.Item(x(i, 1)) = x(i, 2)
Next i

For Each Cel In Rng
    If dict.exists(Cel.Value) Then
        Cel.Value = dict.Item(Cel.Value)
    End If
Next Cel
Application.ScreenUpdating = True
End Sub