First, specify the file you want to upload by following the “specify file for upload” link from your capsule page. A good place to start is “index.gmi”. This takes you to the URL for the new file. It does not exist, yet. In Lagrange, press “Ctrl+U”.
Type your text in the edit area, pick your identity (the same one you use to login), click the upload button or use “Ctrl+Enter”.
Here is how I update my capsule from my phone, with a lot of screenshots: