5 ampere to 15 ampere
Yes but No.

You can plug into a 5A socket an appliance or lead that contains a switch rated at 15A.

You can use a 15A switch to turn on/off an appliance rated at 5A.

However the appliance connected must not draw more than 5A - it's rating must not exceed 5A.
No

You cannot normally increase the rating of a socket without running a thicker wire back to the main fusebox/consumer-unit.