You need to click on the building you would like to upgrade and choose the "Details" option or click the direct "Upgrade" button.

 

In the detail screen you are able to choose to upgrade normally which costs resources and time or instantly by using gold. Once the upgrade has progressed until a certain time threshold, you can use the "Free Finish" option and complete the upgrade right away. 

The "Free Finish" option will usually become available once there are only 5 minutes left on the upgrade timer, however during certain events or with activated VIP-levels it can be used earlier in the upgrade process.
The overall construction duration can be reduced with researches and buffs.