If you are allowed to make a copy, then you can backup computer software by burning a copy of that software from the installation disc provided with the software onto a new physical disc. You should be sure to make this copy directly from the install disc to a new disc, as manually copying the files from one disc to another can corrupt file associations needed to actually perform the installation. If you have install files that you downloaded, rather than an install disc, then you can backup computer software installers simply by copying them to a disc or other format such as an external hard drive or flash drive.
Since you may not be allowed to backup computer software in this way, you may instead need to simply backup your data for future installation. You can save any user data you have generated onto a hard disc or secondary drive, in case your computer or primary drive fails. This allows you to still utilize your personal and user data if you need to create a clean installation of the software. If you wish to backup computer software you have created, then you should typically ensure you have multiple copies of the program on numerous forms of physical media. External storage on a remote server can also provide sufficient backup for your files.