Track changes option

1. At the editor level, in LyX:

   Document > Change Tracking > Track Changes (Ctrl+Shift+E)

2. At the LaTeX level, in any plain text(LaTeX) editor, expliciting manually the type pf changes, with the package ...guess ..., yes, changes.

3. At the plain text level, comparing differences between two versions. There a lot of options here, from control version system, to GUI to compare two files, but the LaTeX related solution here is texdiff.

In this answer there are small examples of use of the three approaches.