Whoops! Did you skip the Uninstall step? It's the same problem if you are given a choice to modify the existing program. In either case, back out and revisit Uninstall.
At 2364 words or 5 1/2 pages, this EULA is shorter than most, but equally boring. Read as much or as little as you please. To continue the installation, you need to click the circle to the left of "I accept the terms...", then click on Next.
No-one will watch over your shoulder to ensure that you fill in your name and that of your organization. Whatever you input will show up here and there later on. When finished, click on Next.
If, and only if, you are installing to the hard disk of your PC, and if you have administrator rights, click Next.
Otherwise, change the destination. If you have not already done so, insert an empty or near empty flash drive [or a portable drive] into a USB port. If necessary, click on the Windows Explorer (or Windows 10 File Explorer) icon and learn the drive letter assigned to your flash drive or portable disk. Click Change in the installation destination screen. Replace the destination with your drive letter, a colon, a backslash, and the word "MarpX", as in "E:\Marpx". Then click on Next.
If you want to change anything in the summary, click Back. Otherwise click on Install.
This error message will show up. It's a plague, a nuisance, and it's wrong! Look up "Installshield 27555" on the Internet to see discussion. A favorite excuse: "A security update introduced by Microsoft on the 14th of July 2015 causes the error."
Solution: Click Ignore. And smile while you do it.
If you have installed on a flash drive or portable disk, it's okay to skip the "verify permission to write" stage. Leave the check mark in place beside "Launch the program". Click Finish.
Security systems are a little unpredictable. If you have installed to the PC, uncheck the check mark and click Finish. Go next to the "verify" stage below.