Batch Replace and add 1 to a number between specific text in an xml file

Ok i'm trying to write a batch to "grab" a serial number for the user. The serial number comes from another system and we have to periodically manually input a serial number. I would like the user to run the batch file which would add 1 and replace the old number and then echo "Your serial number is ##" where ## is the new number. I'm pretty sure i can work out the Echo part and the writing to a temp file and overwriting the old file but its the middle part that im struggling with. The xml file is that is being edited is:

<?xml version="1.0"?>
<BatchSettings xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <SerialFormat>{0:yyMMdd}{1:00}</SerialFormat>
  <LastSerialDate>2020-05-27T00:00:00-05:00</LastSerialDate>
  <LastSerialNumber>10</LastSerialNumber>
</BatchSettings>

the number i need to change is the ## between "lastserialnumber" <LastSerialNumber>10</LastSerialNumber>. I've tried a few different approaches and this first one is close but definitely no cigar.

@echo off
set /a "replace=10"
set /a "replaced=replace+1"

set "source=Test.txt"
set "target=Test2.txt"

setlocal enableDelayedExpansion
(
   for /F "tokens=1* delims=:" %%a in ('findstr /N "^" %source%') do (
      set "line=%%b"
      if defined line set "line=!line:%replace%=%replaced%!"
      echo(!line!
   )
) > %target%
endlocal

The problem with that one is the number won't always be "10" and I also don't want to replace all the other instances of "10" in the file.

I've also tried

@Echo Off
Set "SrcFile=Test.txt"
Set "OutFile=Test2.txt"

@echo off 
    setlocal enableextensions disabledelayedexpansion 

    (for /f "delims=" %%a in (%SrcFile%
    ) do for /f "tokens=2 delims=1234567890 " %%b in ("%%~a"
    ) do if not "%%b"==":"  (
        echo(%%~a
    ) else for /f "tokens=1 delims=  <LastSerialNumber>:" %%c in ("%%~a"
    ) do (
        set /a "n=%%c+1"
        setlocal enabledelayedexpansion
        echo(<LastSerialNumber>!n!:
        endlocal
    )) > %OutFile%

but that doesn't increment the number and cuts off the remaining text. and this one is just a hot mess that doesn't work at all.

@Echo Off
Set "SrcFile=Test.txt"
Set "OutFile=Test2.txt"

setlocal EnableDelayedExpansion

(for /F "delims=" %%a in ('findstr /I /L "<LastSerialNumber>" %SrcFile%') do (
   set "line=%%a
   set "line=!line:*<LastSerialNumber>=!"
   for /F "delims=<" %%b in ("!line!") do echo %%b+1
)) > %oldnumber%

set "newnumber=%oldnumber%+1"

(
   for /F "tokens=1* delims=:" %%a in ('findstr /N "^" %SrcFile%') do (
      set "line=%%b"
      if defined line set "line=!line:%oldnumber%=%newnumber%!"
        echo(!line!
   )
) > %OutFile%
endlocal

At first let us take a quick look at your attempts:

1. fails as it searches just for 10 and does not regard the <LastSerialNumber> tags at all.
2. fails because you try to define a string for the delims= option of for /F although it actually defines a set of characters.
3. fails because you try to redirect (>) into a variable, which cannot work, you can only redirect to files. Furthermore, you attempt to do arithmetics (+1) but this only works when using set /A. And the last section has got the same problem as approach 1.

---

Here is a way you could do it:

@echo off
setlocal EnableExtensions DisableDelayedExpansion

rem // Define constants here:
set "_FILE=%~1" & rem // (input file; `%~1` is the first argument)
set "_TAG=<LastSerialNumber>" & rem // (opening tag)

rem // Loop over all (non-empty) lines of the input file:
for /F "usebackq delims=" %%L in ("%_FILE%") do (
    rem // Store current line string:
    set "LINE=%%L"
    rem // Toggle delayed expansion to avoid troubles with `!`:
    setlocal EnableDelayedExpansion
    rem // Split off opening tag from line string:
    set "TEST=!LINE:*%_TAG%=!"
    rem // Check whether opening tag has been found:
    if not "!TEST!"=="!LINE!" (
        rem // Opening tag found, hence split off closing tag:
        for /F "tokens=1* eol=< delims=< " %%S in ("!TEST!") do (
            rem // Get extracted number and increment it:
            set /A "NUM=%%S+1"
            rem // Return rebuild line with incremented number:
            echo(  !_TAG!!NUM!^<%%T
        )
    ) else (
        rem // Opening tag not found, hence return original line:
        echo(!LINE!
    )
    endlocal
)

endlocal
exit /B

Given the batch file is called inc-serial.bat, call it with the input XML file as a command line argument (say test.xml in the current directory):

inc-serial.bat "test.xml"

To write the output to another file use redirection (>):

inc-serial.bat "test.xml" > "test_NEW.xml"