Hi Mukesh,
To go for SAP BW on HANA certification, note the statement below:-
Copied from https://training.sap.com/
SAP Certified Application Specialist - SAP BW on SAP HANA (Edition 2015)
This certification is issued to candidates that successfully passed the certification exam for the "SAP Certified Application Associate - Modeling and Data Management with SAP BW 7.3 & SAP BI 4.0" (C_TBW55_73) or comparable certification exams (C_TBW45*, P_BIE*, C_TBW60_74).
If you already have BW certifications (any one of those mentioned above), you should be able to just apply for the exam. No experience letter needed.