try this

Code:
Option Explicit

Dim LipidText() As String

Private Sub Form_Load()
    ReDim LipidText(0)

    Call ReadFile(App.Path & "\" & "LipidFile.txt")
    Call ProcessLipid
    Call WriteFile(App.Path & "\" & "LipidFileNew.txt")

End Sub

Private Sub ProcessLipid()
    Dim ComSep() As String
    Dim LPanel() As String
    Dim LipidLine As String
    Dim NewLPanel As String

    Dim i As Long
    
    ' loop through each line in array and process
    For i = 0 To UBound(LipidText)
        LipidLine = LipidText(i)

        ' seperate by comma
        ComSep() = Split(LipidLine, ",")

        ' seperate Lipid Panel text by space
        LPanel() = Split(ComSep(2), " ")

        ' 5 digit text at position 1 of LPanel array
        ' check if we have 8 at the start, if we do - we need to process it

        If Left$(LPanel(1), 1) = "8" Then
            ' add -90 to it
            LPanel(1) = LPanel(1) & "-90"
        End If

        ' join the LPanel text back into ComSep array
        ComSep(2) = Join(LPanel, " ")

        ' restore line and insert into LipidText array
        LipidText(i) = Join(ComSep, ",")

    Next i

End Sub
Private Sub ReadFile(ThisFile As String)
    Dim f As String
    Dim ReadLine As String

    f = FreeFile

    ' Open file to read
    Open ThisFile For Input As #f

    Do Until EOF(f)
        Line Input #f, ReadLine
        ' accept data only, skip empty lines
        If ReadLine <> "" Then
            LipidText(UBound(LipidText)) = ReadLine
            ReDim Preserve LipidText(UBound(LipidText) + 1)
        End If

    Loop

    Close #f
    ' close the file
    
    ' remove extra array record
    ReDim Preserve LipidText(UBound(LipidText) - 1)

    Form1.Caption = "File Read Successful."
End Sub
Private Sub WriteFile(ThisFile As String)
    Dim i As Long
    Dim f As String

    f = FreeFile
    
    ' open the file to write
    Open ThisFile For Output As #f

    ' loop through array and output to file
    For i = 0 To UBound(LipidText)
        Print #f, LipidText(i)
    Next i

    Close #f
    ' close the file
    
    ' delete array
    Erase LipidText

    Form1.Caption = "File write Successful."
End Sub