I want to update the userinfo.php template, but any changes I make to modules/system/templates/system_userinfo.html are not made effective, since the templates were placed into the MySQL database upon installation.

Is there a standard way of making changes to these templates? I could spend a couple hours reading lots of code until I can write the appropriate MySQL commands to update it manually. It would be nice if there is a faster way to get around this problem which seems should be common. Any snippet already in existence?

Thank-you in advance.

There's 2 ways of doing this:

1 - the old method: Clone the template blocks and edit these from the Admin end.

2 - Upload the template you want to change to the themes area, for usage with template-override, which is a standard feature from 2.0.14 upwards.

Example - modules/system/templates/system_userinfo.html would be placed at:
your-site-name/theme/themename/modules/system/templates/system_userinfo.html

The first method worked perfectly! Thank you very much! :)

I would recommend you also learn the second method, which will be more flexible for full control over your site layout.