The templating system for UpStream is much the same as for other plugins such as WooCommerce.
Template files can be found in /wp-content/plugins/upstream/templates/
To override the default templates for UpStream, you need to create a folder within your active theme called /upstream/. Within this folder you can add various files and other folders to overwrite the default templates.
The file and folder structure looks like this:
/upstream
archive-project.php
login.php
single-project.php
/assets
/css
upstream-custom.css
/global
footer.php
header.php
sidebar.php
top-nav.php
/single-project
bugs.php
details.php
discussion.php
files.php
milestones.php
overview.php
tasks.php
You don’t need to add every file and every folder, just the ones that you want to override.
For example, to override the sidebar.php file, you simply copy the wp-content/plugins/upstream/templates/global/sidebar.php file into wp-content/themes/YOURTHEME/upstream/global/sidebar.php and you can then edit this file as you please.
Please note!
Do not edit the files within the core plugin itself as they are overwritten during the upgrade process and any customizations will be lost.