Guidence to upgrade version-14 to version-15

Can any one guide me to upgrade from version-14 to version-15 for production server

I have upgraded my production server. please watch this video.

first do it on a test server with production data.

while upgrading i get this error in switching erpnext branch to v15

INFO: Fetching upstream for erpnext
$ git remote set-branches upstream  '*'
$ git fetch --all --quiet
error: 7291 bytes of body are still expected
fetch-pack: unexpected disconnect while reading sideband packet
fatal: early EOF
fatal: fetch-pack: invalid index-pack output
error: Could not fetch upstream
ERROR: git fetch --all --quiet
subprocess.CalledProcessError: Command 'git fetch --all --quiet' returned non-zero exit status 1.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/bin/bench", line 8, in <module>
  File "/usr/local/lib/python3.10/dist-packages/bench/", line 132, in cli
  File "/usr/local/lib/python3.10/dist-packages/bench/commands/", line 93, in switch_to_branch
    switch_to_branch(branch=branch, apps=list(apps), upgrade=upgrade)
  File "/usr/local/lib/python3.10/dist-packages/bench/utils/", line 108, in switch_to_branch
    switch_branch(branch, apps=apps, bench_path=bench_path, upgrade=upgrade)
  File "/usr/local/lib/python3.10/dist-packages/bench/utils/", line 69, in switch_branch
  File "/usr/local/lib/python3.10/dist-packages/bench/utils/", line 169, in exec_cmd
    raise CommandFailedError(cmd) from subprocess.CalledProcessError(return_code, cmd)
bench.exceptions.CommandFailedError: git fetch --all --quiet

any one have solution ?