I'm not very good with phones so apologies if this is a ridiculous question!

I have the 5c, but I am considering upgrading to the 6. I'm in a contract until February so wondered if it was as simple as purchasing the new handset and putting the sim in? When I swapped from a 4 to a 5 the sim was totally different.

Also my contract doesn't include 4G, just unlimited 3G, so would that be compatible?

Again, apologies if these questions are basic

Just transfer over the SIM card - it is the same size. If you don't have 4G the iPhone 6 will do 3G just fine.

To keep all your Apps and settings (if you are bothered) the Apple approved way is to backup the 5c (to a PC or iCloud) and then restore this backup onto the iPhone 6.

The 6 still uses the same SIM size so no problem there - just simply swap the SIM over. Just do a backup on the 5c (either via iTunes or via iCloud) and then set the 6 up by restoring from the same backup and it will have everything exactly as it was on the 5c.

3G vs 4G won't make any difference in terms of compatibility - the 6 will happily continue on a 3G only contract. Depending on your network it should be pretty straightforward to change to a 4G contract if you wish, and it is unlikely that you will need a different SIM.

It should be fine, assuming the new handset is unlocked or from the same carrier.