To update an existing installation of Bancha, you can simple use Bancha Bash typing on the shell:
To install the bash utilities, see Bancha Bash.
If you doesn’t have shell access, you can do a manual update following this tiny tutorial. Basically, all the Bancha framework is contained inside the core folder.
Beside, all the user content (tipically all the custom things of a website) is contained inside the application folder. This means that to upgrade Bancha, you just need to replace the core folder with an updated one. Finally, overwrite also the themes/admin folder: