Open the dashboard, find the server under My Servers and click Modify. Move the sliders, look at the summary and confirm.
What you pay
The difference between old and new price, prorated for the days you have left. Example: your server has 15 of 30 days left and the upgrade costs €4 more per 30 days, so you pay €2 now, from your balance. The next renewal is charged at the new price.
Downgrades work the same way in reverse: the prorated difference is credited to your balance immediately.
When it takes effect
Immediately for disk and ports. RAM and CPU are applied on the next restart, so restart the server from the panel after upgrading and check the new limit in the panel's resource bar.
What you cannot change
The game type. A Minecraft Paper server cannot become a Rust server; order a new one and move your files if needed. (Switching between Paper, Fabric, Forge and NeoForge is possible: the modpack installer does it for you, or ask Kowalski.)