To add categories to your post tap on your post to launch the Post Options menu and select ‘Edit Settings’ (below)
Alternatively if you’re already editing the post you can tap on the settings icon (top right) to get to your post options (below):
You can see the categories already selected for this post (in this case 1950s, Vintage and Cinema). To add more or edit these tap to view the Category menu. Then just tap each category you want to attach to the post or click the + button to add a new category.