If you're ready right now, go here and click the $79.99 upgrade price. You won't be able to sell or gift PSE 13 afterward because its registration number becomes part of your 2020 upgrade version. Keep both Reg# numbers safe by registering.
Phinny's link takes me to a special offer for $74.99 and it is not for the upgrade which, as he says, is linked to your earlier serial number.
However, just to be clear, the owner of any prior version of Elements is eligible for an upgrade price. It's just that the current full version offer (which expires April 19, 2020) is cheaper than the normal upgrade price.
I highly recommend the full version price currently offered by Adobe at 25% off the normnal price per the link above.
An advantage of the full version is that you only ever need the one version 2020 serial number to reinstall it plus you can sell v13 if you're so inclined or give it to someone else.
With the upgrade, you always need both v13 and v2020 serial numbers to reinstall (people often lose the earlier one) and you cannot sell or donate v13 because the 2020 upgrade version is an extension of your v13 license.