Refreshed Linux Mint Debian Edition (LMDE) 2 'Betsy' ISO images now available

See instruction here:
How to verify a downloaded ISO’s integrity and authenticity
If you find the ISO file to be invalid, most likely there was an error during download. Download again and re-verify.

DVD and USB drive are the most common media on which to place the ISO file.

The easiest way is through the Software Manager: Choose the category, then choose the application you want. At any point, you can use the search feature.
If you just need one or two specific packages, you may want to try the Package Manager (Synaptic).

Both the Software Manager and the Package Manager take packages from the Linux Mint repository. Getting software elsewhere is not recommended unless you trust the source.

Linux Mint, like most distros, handles operating system and application upgrades together.
The easiest way to upgrade is through the Update Manager. The first time Update Manager is run, you choose an update policy based on your comfort level of stability vs. security. (You can always change your update policy options and levels via the Edit | Preferences menu.)

To upgrade, first Choose Refresh, then choose the upgrades you want installed by checking or unchecking the check boxes for the packages you want, and finally choose Install Updates. Update Manager will download, then install the upgrades you have chosen.

“The Update Manager no longer promotes vigilance and selective updates.”[181][184][187] This decimates the Linux Mint update policy and annihilates 12 years of diligence towards responsible and informed system updates. Therefore the following information may be incorrect and/or out-of-date.
Yes, the upgrades are safe. However, since every computer setup is different, no distro can guarantee that all the packages on your system (with your hardware, with your hardware drivers, with your system settings and with your set of applications) are completely compatible with one another. Don’t worry; the vast majority of the time they are.
Linux Mint recognizes the trade-off of stability vs. security and the importance of giving the user a choice. Among other options, Linux Mint allows an informed choice by organizing each package upgrade into levels from 1 to 5. By default, levels 1 and 2 are selected. Choosing upgrades from levels 1 and 2 is a conservative, stable approach; choosing upgrades from only level 1 is a very conservative, stable approach.