You need to comply with all the terms of the EUPL license, which includes the obligations listed in section 5 (partially quoted here):
Attribution right: The Licensee shall keep intact all copyright,
patent or trademarks notices and all notices that refer to the Licence
and to the disclaimer of warranties. The Licensee must include a copy
of such notices and a copy of the Licence with every copy of the Work
he/she distributes or communicates. The Licensee must cause any
Derivative Work to carry prominent notices stating that the Work has
been modified and the date of modification.
Based on the paragraph above you will want to have a section in the UI (maybe called "About"), which includes the attribution information of the original project as well as your own modifications. You might just amend what is already there from the original authors.
Provision of Source Code: When distributing or communicating copies of
the Work, the Licensee will provide a machine-readable copy of the
Source Code or indicate a repository where this Source will be easily
and freely available for as long as the Licensee continues to
distribute or communicate the Work.
Based on the paragraph above you are required to provide the source code of your derivative work. You can do that by providing a link to your website or GitHub repository. This is not optional, you have to provide the source code. If there is enough storage on the Raspberry PI you are shipping, then you may also include the source code in its memory (Flash or thumb drive).