There are a number of ways you can do this:
- Create a template file for user pages in your theme that renders the title differently
- Create a custom module that alters the title using hook_menu_alter
- Use Page Manager (part of ctools) and Panels to customise the layout of the user page
- Likely other ways I don't know about
Number 1 is probably the most common, 2 is only really okay if you're already writing a custom module that changes user page behaviour, 3 is a more advanced method that basically lets you pull apart the page and put it back together how you want.
The simplest version of #3 would be to create a new variant of the user/%uid path with exactly the same content, and just set the title in the page manager UI using tokens.
Searching for keywords from the various options should provide you with plenty of tutorial material.