Can I change a domain's renewal period?

Yes. Every time you generate a renewal proforma invoice from your EUReg customer account, you can choose the number of years you want to renew the domain for — from 1 year up to the maximum allowed by that extension's registry (usually 10 years).

If a proforma invoice has already been generated for a certain period and you want to change it (for example you want 3 years instead of 1), the simplest approach is to cancel the existing proforma invoice (if it hasn't been paid yet) and generate a new one for the period you want, from the same section.

The renewal period doesn't affect the domain's price per year — you simply pay the amount corresponding to the number of years chosen.