v3 as in 3.0?
Since you mention extensions your version should be at least 3.1
And yes it always best to disable extensions before updating AND to check whether they are compatible with the version you're upgrading to.
subsilver was a style for phpBB2 (not even for 3.0) so I suspect you're confusing with subSilver2.
Depending on which version you want to update from the instructions differ slightly.
You're probably wanting to upgrade to 3.3 so then the upgrade instructions are https://www.phpbb.com/support/docs/en/3 ... radeguide/
However I suggest you post the Support Request Template to provide precise information so that advice given is relevant to your phpBB version, and to use a new topic.