If you use BungeeCord, you don't need to take any actions.
(Not recommended) If you use Velocity and you still want to use 'legacy', you need to set 'accessTokenDisabled' to true in your JPremium configuration files on your back-end servers. JPremium won't check any passing data to your back-end servers, so your servers will work in an insecure mode!
Changes:
Added support for BungeeGuard.
Added support for the modern and bungeegaurd Velocity forwarding mode.
Added support for ProtocolLib in version 5.0.0.
Added /forcepurgeuserprofile and /forcemergeuserprofile commands to the Velocity version.
Added ability to disable handshake listeners (the 'accessTokenDisabled' option).
Added ability to define quantity of failed login attempts before disconnection.
Added ability to implement your own resolver using API.
Changed way of registration via a website.
Fixed issues with the "secondLoginCracked" feature.
Fixed HEX colors on a boss bar.
Fixed small issues in the JPremium API.
Removed passing an access token to the handshake on the Velocity version.