I am using
Coveo 4.0 Cloud platform with Sitecore 8.2. I don't see an option for Index Rebuild in Sitecore control panel(Please refer attached screenshot).
Coveo Index can build similar to the Sitecore indexing, in the control panel, under the indexing section click on Indexing Manager. Here you can see the Coveo indexes list with other Sitecore indexes.
Check you Coveo indexes and click on rebuild button.
Update: With the latest version of Coveo you will get the indexing manager, which will give you a UI interface to rebuild the indexes but with your current version Coveo 4.0 Cloud platform with Sitecore 8.2, you will not get this.