You want to update the fundraising thermometer to reflect your fundraising goal for your event.
For walk-a-thons & Pledge events, your event thermometer reflects your pledge drive goal. To change this goal go to Pledge ->Settings. Create/Edit your Fundraising Goal here and your thermometer will reflect this amount.
For other event types, the goal is managed directly in the website page editor. You can add a thermometer or edit the fundraising goal by following the directions found here on fundraising Snippets.