If Column A contains one of several strings then change column Bs string

I am fairly new to macros in excel however I have done programming before. My biggest problem is figuring out the syntax for everything. So wondering if you guys could give me a hand.

What I need is to check to see if a cell in a column matches one of several different strings. If it does then I need to change another cell in the same row but different column to say DELETE. If it does not contain that string then I simple delete it So far I have this

Sub Loop_WIP()
    Dim First As Long
    Dim Last As Long
    Dim Lrow As Long


        First = .UsedRange.Cells(1).Row
        Last = .UsedRange.Rows(.UsedRange.Rows.Count).Row

        For Lrow = Last To First Step -1

            With .Cells(Lrow, "A")

                If Not IsError(.Value) Then

                    If .Value = "ItemA" or "ItemB" or "ItemC" Then

                        Change Value of Column B same Row
                    Else
                        .EntireRow.Delete

                End If

            End With

        Next Lrow

    End With

End Sub

Alternatively I could delete all of the un needed rows then go back with another macro to change all of row B to say DELETED. I still face the problem of not knowing how to format a cell change.

1) Go ahead and break the habit of using .UsedRange in your method for determining the first and/or last row of data. It is full of holes and when it does break you won't know it, you just won't know it's doing wrong things to your data.

Instead, pick the row that always has data filled in, like column A, and just evaluate that column directly.

LR = Range("A" & Rows.Count).End(xlUp).Row

2) Before you run your loop, delete all the "error" rows first to speed up the process, they're no longer there to have to be evaluate. This one line of code will delete every row that has a formula in that results in an error in column A:

Columns("A:A").SpecialCells(xlCellTypeFormulas, 16).EntireRow.Delete xlShiftUp

3) Maybe this then:

Dim r As Long, LR As Long

Columns("A:A").SpecialCells(xlCellTypeFormulas, 16).EntireRow.Delete xlShiftUp
LR = Range("A" & Rows.Count).End(xlUp).Row

For r = LR To 2 Step -1
    Select Case Cells(r, "A").Value
        Case "ItemA", "ItemB", "ItemC"
            Cells(r, "B") = "Here"        'edit this to your own 'change' code
        Case Else
            Rows(r).EntireRow.Delete xlShiftUp
    End Select
Next r

I created a test and made two columns A and B then a filled each row going down with ItemA ItemB etc for 4 rows.

Original
A B
1 ItemA ItemA
2 ItemB ItemB
3 ItemC ItemC
4 ItemD ItemD

Result
A B
1 ItemA ItemA
2 ItemB Here
3 ItemC Here
4

Cells A4 and B4 were both blank i'm suspecting the cost begins 1 cell to late and ends 1 cell to early?

Dim r As Long, LR As Long

Columns("A:A").SpecialCells(xlCellTypeFormulas, 16).EntireRow.Delete xlShiftUp 'didn't like this to much came back with an error that no cells were found'
LR = Range("A" & Rows.Count).End(xlUp).Row

For r = LR To 2 Step -1
    Select Case Cells(r, "A").Value
        Case "ItemA", "ItemB", "ItemC"
            Cells(r, "B") = "Here"        'edit this to your own 'change' code
        Case Else
            Rows(r).EntireRow.Delete xlShiftUp
    End Select
Next r

Most people have some sort of "titles" in row 1, if you don't it's a simple adjustment:

For r = LR To 1 Step -1

You use a space-underscore combination to break long strings.

Dim r As Long, LR As Long

Columns("A:A").SpecialCells(xlCellTypeFormulas, 16).EntireRow.Delete xlShiftUp 
LR = Range("A" & Rows.Count).End(xlUp).Row

For r = LR To 2 Step -1
    Select Case Cells(r, "A").Value
        Case "ItemA", "ItemB", "ItemC", _
           "ItemD", "ItemE", "ItemF"
            Cells(r, "B") = "Here"
        Case Else
            Rows(r).EntireRow.Delete xlShiftUp
    End Select
Next r