The current max level for the workbench in Valheim is level 5. Just follow this guide to update the workbench to its max level (Level 5). I hope this will help you and other players to achieve the max workbench level in Valheim.

• Simply build a workbench which will be on level 1.
• Build the Chopping Block extension to update the workbench to level 2. The chopping block is unlocked until you find wood and flint. Both can be found in the grasland. Just pick up some wood stick from the ground. Flint can be found directly on the water. Flint looks like small shaved stoned. 
• Build the Tanning Rack extension to update the workbench to level 3.The tanning rack is unlocked after you found Flint, Leather Scraps, and Deer Hide.Go for hunting in the graslands. Hunt some Deers and Boar to find those ressources.
• Add the Adze extension to update the workbench to level 4.The Adze is unlocked after you picked up Fine Wood and Bronze. You need a Forge near in order to place it down.
• Build the Tool Shelf to update the workbench to level 5. The tool shelf will be unlocked after you picked up Obsidian.Obsidian can be found in the snowy mountains. You will need a metal pick axt to harvest obsidian. 
• Workbench to level 6 is not available yet but will be added to the game later.