I recently forked a project and applied several fixes. I then created a pull request which was then accepted.
A few days later another change was made by another con
That depends on the size of your repository and how you forked it.
If it's quite a big repository you may have wanted to manage it in a special way (e.g. drop history). Basically, you can get differences between current and upstream versions, commit them and then cherry pick back to master.
Try reading this one. It describes how to handle big Git repositories and how to upstream them with latest changes.