Need help replacing values in multiple cells

Well, it works on your sample

Sub PhilW(): Dim UR As Range, n As Long, m As Long
Dim C As Range, S As String, Q As String
Set UR = ActiveSheet.UsedRange
For Each C In UR: S = C.Value
n = InStr(1, S, "Birthday")
If n = 0 Then GoTo NextCell
m = InStrRev(S, " ", n - 3) + 1
Q = Mid(S, m, n - m - 3)
If IsNumeric(Q) Then
Mid(S, m, n - m - 3 + 1) = CStr(Val(Q) + 1)
C.Value = S: End If
NextCell: Next
UR.Replace "0h", "0th"
UR.Replace "1th", "1st"
UR.Replace "2th", "2nd"
UR.Replace "3th", "3rd"
UR.Replace "2st", "2nd"
UR.Replace "3nd", "3rd"
UR.Replace "4rd", "4th"
End Sub

Directions for running the routine(s) just supplied

If you haven't used macros before you'll need to go to:
File- options - trust center -trust center settings - macro settings ,
the second option down (disable all macros with notification)

Then - Copy the code to the clipboard

Open your Workbook

Press ALT + F11 to open the Visual Basic Editor.

Select "Module" from the Insert menu

Type "Option Explicit" then paste the code under it

With the cursor between Sub and End Sub press F5 (F8 to Single Step)

OR

Press ALT + Q to close the code window.

Press ALT + F8 then double click on the macro name
*Be sure to save the book with the code as Macro-Enabled