Yes, you can change, modify, edit or delete content in a proposal once it's sent. To do that you will need to withdraw your proposal, then fix the error and resubmit it. If you do that, you will Not get back the connects that it "cost" to send the original proposal And it will cost you another 2 connects to send the "fixed" proposal.
You are applying for jobs in Graphic Design.
I doubt any of your prospective clients will care very much about a typo.
If you were applying to do graphic design for me, I can assure you that I would not notice.
I don't read proposals from graphic designers. I just look at their portfolios.