my last activity on the project was finishing lets say on November 1. When I put a finish no later than constraint to the last activity as something before november 1, such as October 31, it reduces the toatl float by 1 day and makes the total float of critical path as -1, which i understand

but when i put a finish no later than constraint to that activity as november 2, nothing changes. should this not increase the total float of critical path?

I'm guessing you mean the total slack returns to zero? FNLT constraints (as long as they are not violated) will not add positive total slack. You are merely saying the task needs to finish by a certain date - but it can finish earlier. If you wish to add total slack - set a must finish on.