Your Energy Jar, displayed on the top of the screen, is an artifact that increases the capacity of your Energy Bar.
The numbers displayed indicate the amount of Energy you currently have (on the left) and the maximum amount your Energy Bar can hold (on the right).
When you collect enough Magic Items for the upgrade, you'll see a popup reminding you an Upgrade is ready.
To upgrade your Energy Jar
:
1. Click on the Artifacts icon at the bottom of your screen
2. Find the Energy Jar on the list of Artifacts.
3. Check whether you've collected enough Magic Items and Gold to upgrade.
When the Artifact is ready to be upgraded, green arrows
will appear next to it. 
4. Click on the green button to complete the upgrade.
💡Learn more about: getting more Energy and Unlimited Energy