Change start time of hourly candles

This is a chart setting. The setting is called "Start aggregation at market open". Screenshot below shows where to find it.

Very important to note that doing this to your chart will ensure that it will no longer match custom watchlist columns, scans, study alerts or conditional orders. If you don't use any of those other features there is no harm is setting your chart to start aggregation at market open. Until Thinkorswim developers add this same setting to custom watchlist columns, scans, study alerts or conditional orders it is recommending you NOT use this setting.

Notice the box in the screenshot below is UNCHECKED as this makes your charts compatible with the rest of the tools on the platform is therefore the preferred setting. If you want to accomplish exactly what you have requested, then you will actually check that box instead of uncheck it.