Can I edit my timer after sending the campaign?

Yes!

A big advantage of Mailtimer over other timers is that it supports real-time updates. A change in the deadline? Edit the timer on-the-fly, even if it has already been sent.

Next time a recipient opens the email, a new (updated) timer will be generated and shown in the email.

Note however, that some email clients may cache the image content and therefore some of your recipients may still see the previous version of the timer.