How to Change the Boundary Value of my FE space directly in FreeFem++?

Question

I'm working on a FSI problem with FreeFEM++ recently.

My model is a 2d fluid model coupled with a 1d elastic model, and I seem to work well with the 2 parts individually.

Then I met a problem when I decide to couple them together: I have read the value on the boundary part of my 2d FE space and I have updated them by my 1d model, but I don't know how to modify the 2d FE space directly.

Or where can I found the introduction about the structure of some specific FE space?

Any piece of advice will be appreciated.

Thanks!