How can I uninstall yosemite and reinstall Loin with out losing all my in formation and programs

Question

How can I uninstall Yosemite and reinstall Loin with out losing all my in formation and programsThis had been the worst os system apple has come out, too many errors
too many to list.
a few
Losing all mail. and watching them disappear in front of my eyes and could nothing about it
also when accessing cloud the screen just rolls in when accessing mail, like loading and loading and loading for 1 to 2 minuets.
I did not up grade my Mac Book Pro, and is working fine,
Just my iMac, its act kile avid card and it only happens access iCloud mail.
so I want to and need to go back to Loin

Answer 1

You can't unless you have a current backup of your old Lion system because the first thing you must do is erase the drive. It 's possible to use Target Disk Mode between your iMac and the MBP in order to clone the MBP to the iMac. However, that will still erase everything on the iMac's drive.
Or you could see about getting Yosemite to work correctly. If you have IMAP mail, then your mail should be intact.
Try these in order testing your system after each to see if it's back to normal:
1. a. Resetting your Mac's PRAM and NVRAM
    b. Intel-based Macs: Resetting the System Management Controller (SMC)
2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the Utilities menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu. Select Restart from the Apple menu.
4. Reinstall Yosemite: Reboot from the Recovery HD. Select Reinstall OS X from the Utilities menu, and click on the Continue button.
Download and install the 10.10.1 update.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
            because it is three times faster than wireless.
5. Reinstall Yosemite from Scratch:
Download and install the 10.10.1 update.
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
How to Clean Install OS X Yosemite
Note: You will need an active Internet connection. I suggest using Ethernet if possible
            because it is three times faster than wireless.