Themes don't refresh when i change them

Hello,

Here’s my problem, when I change a template (theme), it does not become updated in the browser. It’s always the same which appears and never the one i activate. It do that with every theme i try. It happens the same on the 3 browser i’v tried : Chrome, Firefox and Opera. All plugins are disabled. Sometimes it changes, but after 2 hours !
If you know where is problem…

Thanx a lot !