When editing a client account in Contact Reach, you'll usually encounter 3 Update Types in Step 2:
Trial - a package update type that automatically gets assigned to a client account when you create it for the first time. You won't be able to find the package update types selection when creating a client account. But when you edit the client account, that's the time you'll find the selection in Step 2.
The purpose of the Trial update type is to let you assign another package with a "trial period" to the client account - for instance if you've mistakenly selected a wrong package and wanted to assign the correct package that includes a trial period.
Upgrade - this package update type allows you to upgrade the package to another package. When you choose "Upgrade" and select another package, the credits from the original package of the client account will remain and be added to the credits of the other package that you upgraded it to.
Reset - this package update type allows you to reset the client account's package to another package. This is what you can use if you mistakenly assigned the wrong package to the client account and you want to assign the correct package afterward. This works almost the same as the Trial update type but it "bypasses" the trial period of the package that you assigned it to. So only the actual package credits will show and be used.
Here's a video that you can watch that also explains the same thing above: https://www.screencast.com/t/3yc1MIHcseZ (the video addresses Michael who's a customer whom I attended at the time that I recorded the video)