Pull Request - fixing "This branch has conflicts that must be resolved"

Hi

I have forked a project I want to use/change - https://github.com/tjscience/RoboSharp/tree/master

I then compare it with another fork which has some new features in it which are not in the original project - https://github.com/alex-becker-startp/RoboSharp/tree/new/copyResults

I then click Create Pull Request twice

I am then told the following:-

This branch has conflicts that must be resolved

Use the command line to resolve conflicts before continuing.

Conflicting files
RoboSharp/RoboSharp.csproj

Please can someone tell me how to fix this so I can merge changes

I only ever use GitHub via website, so no idea how to use command line to fix it?

I tried renaming that file in my fork first but then still get same error

Thanks you

For info - I am using Windows 10 - and would prefer to know how to fix via website or github desktop if possible? unless via command line is only option