Upgrading portals is essential to increasing their defensive capabilities as well as their linking capabilities. You can also use the upgrade menu to deploy higher level portals directly to empty slots within a portal and control where which level resonators go on the map.
To upgrade a portal you have to deploy higher level resonators on it. The portal level is calculated by the average all resonators deployed on it. So if you have 7 level 1 resonators and 1 level 3 resonator your portal level still be level 1. Learn more about portal levels in Leveling Portals.
You can also upgrade a portal by deploying a portal shield to it. These do not help the portal level but will help it defensiveness via damage mitigation should come under attack.
To bring up the upgrade menu you need to be within action range of the friendly portal (45m) and click on it to bring up the portal menu. From the portal menu click on the Upgrade button to bring up the Upgrade menu. Once you have the Upgrade menu open you can select any resonator location and click deploy to select a level of resonator to deploy to that slot. You can see information on which resonator you have selected to determine where it will be placed in relation to the portal. Then you can click on one of the 4 portal mod locations to select a mod to deploy to the portal.
Note: You only gain AP when deploying to an empty slot. So if you are upgrading a resonator or shield you will only gain the Action Points if the slot is empty.
Tip: Be aware that the distance you are standing away from a portal will determine the distance from the portal that the resonators are placed. So to increase the difficulty in attacking a portal it is important to stand as far away from a portal as possible while upgrade/deploying.
Trick: Deploy higher level resonators to harder to reach locations in order to increase the difficulty in destroying them.