Whilst I'm not bob
I would suggest that you first download the Avast off-line installation file and save it somewhere you can find it again. So you don't have to go online without protection. Essentially downloading and saving all files whilst you are protected by your existing program.
You don't say what avast Program you are considering or I would try to give you a link, but I'm sure you should be able to find it in the forums
Having saved the Avast off-line installation file, you can now uninstall McAfee using the normal Windows Programs > Uninstall function, restart and then install Avast and again restart.
There are some that would consider downloading the specialist McAfee uninstall tool to ensure absolutely everything is removed.-
https://service.mcafee.com/?articleId=TS101331&page=shell&shell=article-view - Again downloading the removal tool and save it so you don't have to go online without protection.