How do I get iOS 5

if you have laptop/desktop computer you can sync the iPad with iTunes and download iOS 8.1.2 there:
Upgrading iPad to iOS 8: Through iTunes 1. Backup your iPad
Open iTunes. Select your iPad in iTunes and click Back Up Now.
2. Restart your iPad and your Computer
3. Download and Install the Update
Go to iTunes -> Select iPad -> Check for Update -> Click Download and Update

Whichever method you choose, the process will take quite some time to work depending on how much free storage you have on your iPad, your internet connection, and how Apple's servers are doing with upgrades. Occasionally, Apple will either not let you download the upgrade or it will fail. For iOS 8, you must have an iPad 3 or newer. This does not work with the original iPad - if something goes wrong, restart your device, and try again.

How to Upgrade to iOS 7 on an iPad or iPad Mini
How to Install iOS 8 When Your Device is Low on Storage

no
- if you have an Ipad first gen. it is impossible to update to iOS 6 as it is not compatible. First gen iPad not getting iOS 6 update Apple confirms

the first gen. iPad is not compatible with iOS 6
- you need a newer one....
Apple iOS 6 Finally Available for Download Direct Download Link Download Apple iOS 6

iOs 6 is not compatible with the first gen iPad:
Apple iOS 6 Finally Available for Download Direct Download Link Download Apple iOS 6

you cannot install iOS 6 because your iPad is too old:
For the iPad first generation (A1219) to update is only possible with connecting the device to iTunes on a computer.
- this model supports only iOS 5 - NO iOS 6/7/8

if you have a first gen. iPad you cannot update to iOS 6 as it is only compatible to iPad2 and newer:
First gen iPad not getting iOS 6 update Apple confirms

no, if it is a first gen. iPad it is not compatible with iOS 6:
First gen iPad not getting iOS 6 update Apple confirms

NO
- iOS 6 and higher is not compatible with first gen. iPads!
First gen iPad not getting iOS 6 update Apple confirms

First Generation iPads can use iOS 5
- but iOS 6 / 7 / 8 need an iPad 2 or newer and are not compatibel with first gen. iPhone
-updates can be made in iTunes connected to a computer or under settings - general - software update
Upgrade iOS from within iOS device
(valid for iOS 5 and above)
Since iOS 5, it is possible to upgrade the iOS version without the need to connect the iOS device to a computer (over-the-air update). To upgrade to iOS 6/7/8, simply navigate to Settings > General > Software Update. The screenshot below demonstrates how to do so on an iPad, but this is also valid for the iPhone and iPod Touch.

in iTunes:
How to update to iOS 7

The ipad 1 will not update to ios 7 because it will make it run too slow, and u don't want it at the speed it will run, apple decided to not let the ipad 1 work on ios 7 because of the speed and it would also make it crash heaps.