Are your dentures new or old?
when was it made?
Are these complete dentures or partial dentures?
You will have to visit your dentist to get this denture corrected.
It is a complete denture and you should be able to take them out.
Rinsing the mouth with warm water [forcefully] could loosen the denture.Try to loosen the denture ,without applying too much pressure,by using your finger on the edges of the denture,the area where the denture meets your gums.
There is a possiblity that the denture is stuck in the undercuts in your upper jaw bone,because of the relining .If it is stuck ,then you will have to visit your dentist to get it removed.
You will have to visit your dentist to get this denture corrected so that it doesnt trouble you in future.