https://www.epo.org/en/node/epo-international-searching-0

How can I file subsequently filed documents with the EPO as ISA in the pilot?

You can submit subsequently filed documents (SFDs) to the EPO as ISA (e.g. rectification of obvious mistakes in the description, claims or drawings under Rule 91 PCT) by post or fax, or via the EPO's online filing tools (Web-Form Filing, Online Filing (OLF) or Online Filing 2.0 ).

Alternatively, you are encouraged to use WIPO's ePCT service to file SFDs or any PCT Chapter II demand. The ePCT service can also be used for indicating the payment of fees related to the demand (see OJ EPO 2016, A78).