Is your mattress still in good condition?
Consider reuse! If your mattress is still in good condition, it may be of use to someone else, in which case, a second-hand shop will be happy to take it.
Give them a visit as they are best placed to determine whether your mattress is still reusable.
Contact a thrift shop in your areaIs your mattress in poor condition?
If your mattress is stained or its seams are damaged, you will need to take it to a recycling centre. Make sure it is dry and whole to ensure it can be recycled.