This is not possible by yourself. SAP steers that. At the moment there is no official timeline if and when SPS09 or SPS10 will be applied to the trial instance.
If you wanna, you can use the SAP HANA Developer Edition from SAP Cloud Appliance Library. This image can be used on AWS. For HANA you do not have to pay a licencse, but you need to pay the AWS time. At the moment there is a SPS09 version available of the HANA Developer Edition. A SPS10 version should follow until the end of the year.