Upgrading and patching your company instance requires planning, testing, and validation. you need to create upgrade plans, and test your upgrade on non-production instances before upgrading your production instance. For Washington Release, please see How to upgrade your PDI to Washington DC.

However for your Personal Development Instance (PDI) you can easily follow the following steps:

1. Login in developer.servicenow.com site
2. Click down allow near your name on top right
3. My instance information menu is displayed
4. Click "Upgrade Instance" and then a list of instance versions are displayed
5. Select "Vancouver" and then click "Upgrade Instance" button
6. Upgrade is in Progress

The upgrade will take around 2-3 hours to be completed. You shall received the confirmation email -

Reference

How to upgrade your PDI to Washington DC