Q: How do I upgrade OS x without a credit card?

Upgrading from Mountain Lion to El Capitan is free of charge, one of the choices that should be listed when asked for payment details is 'None'. If you select None as the payment type and then 'buy' El Capitan because it is free it should allow you to do this.

LauLauRSA wrote:

 

Hello, I need to upgrade my OS X but do not own a credit card and the Apple store won't accept my debit card details, although it is a MasterCard. Any suggestions are welcome please.

You don't need a credit card to upgrade to El Capitan from Mountain Lion.  All of the upgrades since Mavericks have been free.

Where are you trying to download El Capitan from?  Are you signed into the App Store?  I've never been asked to input any credit/debit card details before upgrading my OS.

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

Once you've created an App Store account without entering a credit card, you will be able to download any item that is free (this includes freemium items, which aren't really free).

If you already have an account, it shouldn't matter at all with El Capitan. At no point whatsoever should it be asking for credit card details to download it.

Before you upgrade, make sure your printer and any version of Office you have is compatible, and backup your data:

http://discussions.apple.com/docs/DOC-8811

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

[edit]

Kurt beat me to this with the same link.

LauLauRSA wrote:

 

Yes from within App Store - under software updates?

If your upgrading from Mountain Lion to El Capitan it is not an update it is an upgrade.

There is a meaningful difference between an update and an upgrade. An update is usually taken to mean a small version change e.g. 10.8.4 to 10.8.5, an upgrade is usually taken to mean a big change e.g. 10.8.5 to 10.11.3 as would currently be the case for upgrading from Mountain Lion to El Capitan.

As it happens both OS X update and OS X upgrades are free of charge. However to upgrade from Mountain Lion to El Capitan you need to use the search box in the top right search for El Capitan. It should then be the first result listed.

It will not let me select none as an option.

You might want to consider starting a new discussion. Since this one is marked solved, less people are likely to look at it. You can link to this one.