UserForm to add from a selected cell from a TextBox value - excel

Question

I am trying to have a UserForm GUI so that we can add and subtract from inventory, I have got it so that I can select a worksheet and a row, but I am having trouble adding and subtracting part. Pretty new to VBA and I am not sure how to call that variable and modify it. Any help would be great!! Here is my code in the UserForm:
Option Explicit
Private Sub BTNadd_Click()
End Sub
Private Sub BTNDone_Click()
'This will save and close the GUI'
ThisWorkbook.Save
StgRmGUI.Hide
End Sub
Private Sub BTNrmv_Click()
End Sub
Private Sub ItmNmSlct_Change()
Dim actItm As String
End Sub
Private Sub ItmTypSlct_Change()
'This allows ItmTypSlct to show available wrkshts then will make item show in Item Name box'
With Worksheets(ItmTypSlct.Value)
ItmNmSlct.RowSource = Range(.Cells(2, 1), .Cells(Rows.Count, 1).End(xlUp)).Address(, , , True)
End With
End Sub
Private Sub NumBox_Change()
Dim NewVal As Integer
NewVal = Val(NumBox.Text)
If NewVal >= SpBtnARNum.Min And _
NewVal <= SpBtnARNum.Max Then _
SpBtnARNum.Value = NewVal
End Sub
Private Sub SpBtnARNum_Change()
NumBox.Text = SpBtnARNum.Value
End Sub
Private Sub UserForm_Click()
End Sub
Private Sub UserForm_Initialize()
Dim ws As Worksheet
For Each ws In ActiveWorkbook.Worksheets
If ws.Visible = xlSheetVisible Then
ItmTypSlct.AddItem ws.Name
End If
Next ws
End Sub

Answer 1

An simple example of updating the column B value by the amount in NumBox
Private Sub BTNadd_Click()
Dim r As Long, cell As Range
With ItmNmSlct
r = .ListIndex
If r < 0 Then Exit Sub
' select quatity cell and increment value
Set cell = Range(.RowSource).Cells(r + 1, 2)
cell.Value = cell.Value + NumBox.Value
End With
End Sub