Windows 3.11 is a DOS application. It is not an Operation System like 95/98/XP.
You probably are working with a computer with the 386 or 486 system. You are also probably working with SIMM RAM. SIMM RAM is very hard to locate, and you may only be able to upgrade your SIMM RAM up to 64 MB max. Both Windows 95 and 98 need a bit more RAM to run efficiently than the Windows 3.11. You are also running DOS 6.x or less. If you need to go back to 3.11, you will not be able to unless you have DOS to reload.
Also, your Windows 95 may be Floppy Disk. It is rare to find the 95 that came on the CD, but it will have some of the updates needed to stablize Windows 95.
What you do is to save to a Floppy disk ALL files, pictures and other personal information you don't want to lose.
Insert the startup disk for Windows 95 and follow the directions.
I would advise you to get Windows 98SE. It is similar to Windows 95, but much more stable. Basically, Windows 98SE is Windows 95 with all the updates needed to fix Windows 95. And, they both can work on the same types of PC's.
As mentioned.... Windows 3.x, 95, 98 and NT are no longer supported by Microsoft. If you get virus and Trojan problems with these OSes, you will not be able to get fixes to the problems.