In addition to kus answer.
There are some changes in Xcode 8.0
Step 1:
Change scheme destination to Generic IOS device
.
Step 2:
Click Product
> Archive
> once this is complete open up the Organiser and click the latest version.
Step 3:
Click on Export...
option from right side of organiser window.
Step 4: Select a method for export > Choose correct signing > Save to Destination.
Xcode 10.0
Step 3: From Right Side Panel Click on Distribute App.
Step 4: Select Method of distribution and click next.
Step 5: It Opens up distribution option window. Select All compatible device variants and click next.
Step 6: Choose signing certificate.
Step 7: It will open up Preparing archive for distribution window. it takes few min.
Step 8: It will open up Archives window. Click on export and save it.