Step 1. Go to Catalog > Products and select product to Edit.
Step 2. In Related Products, Up-Sells, and Cross-Sells tab you will find three sections where you can Add the products for each section.
Step 3. Save the product.
Step 4. Go to System > Index management and if necessary reindex your catalog. More details on how to reindex your store here: How to reindex your Magento 2 store from admin or SSH / CLI
Step 5. Check the store front, make sure you are visiting the product where you made the changes. If the Related / Up-sells / Cross-sells are not showing up at this point, try clearing Magento cache from System > Cache management > Flush Magento Cache or other 3rd party caches that you are using.