Download the plugin's source code Javascript and CSS (or less) and replace old class references to new, see: http://www.bootply.com/bootstrap-3-migration-guide.
B.e. as mentioned by PilHA replace icon-*
with glyphicon glyphicon-*
, input-append
with input-group
etc.
Do the same for your HTML code. Or use a migrator/updater like: http://twitterbootstrapmigrator.w3masters.nl/, http://bootstrap3.kissr.com/ or http://code.divshot.com/bootstrap3_upgrader/
Install Glyphicons from http://glyphicons.getbootstrap.com/: download the files and copy over all the font files to a /fonts directory near your CSS. Include the compiled CSS file from the /css in the repository to your local css folder or download the Less file and compile it with your Bootstrap files.
update Glyphicons are back since RC2. Twitter's Bootstrap 3 includes 180 glyphs in font format from the Glyphicon Halflings set.