Is it possible to update the candy machine with adding new nft to the collection ?
I mean I want to release the web site by July but the mint will only take place in september so in July I will not have the entire collection finished... Is it possible to deploy the candy machine then add the nft assets (png + json files) ?
I didn't find anything about it on the metaplex doc...
Once a candymachine is uploaded and written to it can't be uploaded to again.
You could make another candymachine on the same keypair/wallet address and merge the collections with a site over at https://collections.metaplex.com/. This site allows you to move NFT's you have update authority over to either a new or existing collection you own. So you'd just need to provide a hashlist (token addressses) of all the NFT's you want to move to a collection.
I would suggest either waiting and uploading everything at once or you can go through the process of merging them all to the same collection after. It just means you'll have to run 2 candymachines for 2 seperate mints. Need to decide whats the better look for your community.
Hope that helps :)