-1. Download the 5022 version: http://usdl.synology.com/download/DSM/5.1/5022/
0. rename "DSM_DS3615xs_5022.pat" to "DSM_DS3615xs_5700.pat"
1. start your NAS in "Install/Upgrade" mode
2. open the web console
3. update manually, select "DSM_DS3615xs_5700.pat" file
(Resets the config only.)