Overwrite single file in my current branch with the same file in the main branch?

Question

git checkout master path/to/default.aspx.cs

Before doing this, you probably have to : git checkout redesign

So, just git checkout FROM_BRANCH_NAME path/to/file

To overwrite a file in a branch from another branch, i.e, master to redesign do (when redesign is the current branch)

git checkout master ./path_to_file/default.aspx.cs

git checkout branch_name ./path_to_file/file_name.[file_extension]

To know more about the command checkout git checkout documentation