The normal process to following when performing an upgrade:

1) Backup your site
2) Uninstall the MOStlyCE Admin component
3) Uninstall the MOStlyCE mambot
4) open your file manager and make sure the following files and directories are not present and if they are then remove them manually:

• administrator /components /com_mostlyce
• mambots/editors/mostlyce
• mambots/editors/mostlyce.xml
• mambots/editors/mostlyce.php
• components/com_mostlyce_frontend

5) Install the new MOStlyCE mambot
6) Install the new MOStlyCE component
7) Go to Site->Global Configuration. Change the WYSIWYG Editor drop down to "No WYSIWYG Editor", hit apply, and then change it back to MOStlyCE and hit Save. This is needed to trigger the change otherwise you may simply see a blank text area where the editor use to be.
8) Clear your browsers cache
9) Pull up a content item in the editor, click on the editors help icon (the question mark button), and verify that you have the same version of TinyMCE stated in the MOStlyCE release notes. Each new version of MOStlyCE tries to contain the latest version of TinyMCE available at the time.