Please perform the below steps prior to updating to ensure you have a smooth experience:
Review the Order Enhancements Changes Overview guide in the Fonteva 20Spring Release Notes to see if any customizations in your org are still viable. If you have any customizations on the Invoice object, you will need to move those to the Sales Order object for Posting Entity: Invoice Records. We recommend using a sandbox environment to test changes to customizations before upgrading in your working environment.
Navigate to Setup > Company Settings > My Domain and select Edit under My Domain Details.
Deselect Stabilize Visualforce, Experience Builder, Site.com Studio, and content file URLs. The option Include the instance name in Visualforce URLs when third-party cookies are blocked disappears when Stabilize Visualforce, Experience Builder, Site.com Studio, and content file URLs is selected.
Select Save to apply changes.
Navigate to Setup > Apex Settings, and make sure Deploy Metadata from Non-Certified Package Version via Apex checkbox is enabled.
Follow up with Part 2: Updating.