You code looks like you copied code parts together, correct me if I am wrong. Therefore I applied the change you required and commented and renamed parts in the code for better understanding. If you dont find this usefull, please comment and I will delete the answer :)
Sub simpleXlsMerger()
Application.ScreenUpdating = False
'Define a Workbook Object for the Workbooks to pull data from
Dim sourceWorkbook As Workbook
'Define a Workbook Object of the Workbook to summarize data in
Dim summaryWorkbook As Workbook
'Set the summary Workbook to ThisWorkbook
Set summaryWorkbook = ThisWorkbook
Dim folderObject As Object
Dim myFolder As Object
Dim myFiles As Object
Dim everyObj As Object
Dim folderPath As String
folderPath = "C:\Users\5545\Desktop\MI"
Set folderObject = CreateObject("Scripting.FileSystemObject")
Set myFolder = folderObject.Getfolder("C:\Users\5545\Desktop\MI")
Set myFiles = myFolder.Files
'For each file in your directory you set the sourceWorkbook equal to that file
For Each File In myFiles
Set sourceWorkbook = Workbooks.Open(File)
'You define the range within this Workbook and copy the range
sourceWorkbook.Range("A2:IV" & Range("A28").End(xlUp).Row).Copy
'You append the summary sheet with the copied data
summaryWorkbook.Range("A65536").End(xlUp).Offset(1, 0).PasteSpecial
'You close the file and open the next in the next loop
sourceWorkbook.Close
Next
summaryWorkbook.Activate
End Sub
General remark for beginners to access Workbooks, Sheets and Ranges:
'Define an object container for workbooks,sheets and ranges you work with
Dim wb As Workbook
Dim ws As Worksheet
Dim rng As Range
'A worksheet can be assigned by Name when its open, set to the Workbook the code is within, open a Workbook by Path or add a new one
'The workbook can then be accessed by using the variable e.g. wb
Set wb = ThisWorkbook
Set wb = Workbooks("WorkbookName")
Set wb = Workbooks.Open("FilePathIncludingNameAndFileType")
Set wb = Workbooks.Add
'Similarly worksheet objects can be assigned by Name, Index or added. Dont forget to use wb. to reference in which workbook to look for the sheet
Set ws = wb.Sheets("NameOfSheet")
Set ws = wb.Sheets(IndexOfSheet)
Set ws = wb.Sheets.Add
'Ranges are Objects within Worksheets and can be accessed by A1 notation e.g. "A1:C10" or by cell reference e.g. (Cells(1,1), Cells(10,3))
Set rng = ws.Range("A1NotationOfRange")
Set rng = ws.Range(Cells(RowIndex, ColumnIndex), Cells(RowIndex, ColumnIndex))