Many buildings in your yard can be upgraded. When you upgrade a building, the building will function better than before or open up new possibilities. For example, resource buildings will start producing more resources and will be able to hold more of them in storage. Likewise, Storage Silos would be able to hold more resources. Upgraded defensive buildings may increase in damage per second and tower range.
As your base grows, you will find that you will need to upgrade your Town Hall to unlock more buildings and to unlock higher level building upgrades.
A building that is Upgrading can not be moved at all by clicking on the building since the move button is not available for you. But, if you use the new Yard Planner 2. You are able to move buildings that are upgrading freely.
Friends can help speed up upgrading buildings. Every time a friend helps you, 5% of the time currently left to complete the building is taken off. Only a maximum of 5 friends can help on each building upgrade, reducing the time for the upgrade by about 25%. (An upgrade that takes 1 day can be reduced to about 18 and a half hours, not 18 hours. This is because the theoretical maximum total reduction is calculated by (1-(1-.05)^5) which equals ~22.62190625%)
If monsters or resource catapults destory a build that is upgrading, the building will increase upgrading time by 5%.It is recommended that you should upgrade defensive towers when you either have protection time or other buildings protecting it.