tpm/docs/changing_plugins_install_dir.md

Changing plugins install dir

By default, TPM installs plugins in a subfolder named plugins/ inside $XDG_CONFIG_HOME/tmux/ if a tmux.conf file was found at that location, or inside ~/.tmux/ otherwise.

You can change the install path by putting this in .tmux.conf:

set-environment -g TMUX_PLUGIN_MANAGER_PATH '/some/other/path/'

Tmux plugin manager initialization in .tmux.conf should also be updated:

# initializes TMUX plugin manager in a new path
run /some/other/path/tpm/tpm

Please make sure that the run line is at the very bottom of .tmux.conf.