I can't download apps as I need to update the software

An iPhone 6 can't be updated beyond 12.5.7, it doesn't have the hardware required to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.2

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.6. 1

You cannot update an iPhone 6 to iOS 13 or higher.

Many modern apps require something better than iOS 12 and thus are incompatible with the iPhone 6. If you need or want to run them, you will need to get a new (or newer) iPhone. Preferably one that will be compatible with iOS 18 (when it comes out).

Settings > General > Software Update

If you are tempted to buy a used iPhone to "save money", first read

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community

so that you will know what to look for to avoid buying an Activation-Locked and possibly stolen phone.

You can find a list of iPhones that will be compatible with iOS 18 near the bottom of this page:

iOS 18 Preview - Apple

Note that some of the phones on that list do not support 5G cellular networks. You can double-check, but I believe you need an iPhone 12-series or newer phone for 5G support.