At the left side in Menu Bar, there is option of button "Setting". You can find setting option in "My account" as well. We have to click on it.
Now we will go in the "Quotation" and we have to click on the "Quotation Number". Then 2 options will come.
1. Manual
2. Automatic
Manual - In Manual option, we can give our own quotation number manually while creating every quotation.
Automatic - In Automatic option, it will give quotation number automatically. We can change the 'prefix' according to us.
And then click on the button "Update" to save the settings.