hi,

yes, you can again join as a external candidate in board of education. however the best option for you is National institute of open schooling. nios certificate is accepted by most of the tier 1 government and private institute and organization In india. however NIOS has many branches, you may take admission in the institute which are located near your location. There is no upper age limit for admission to nios . However, the minimum age for enrollment is 14 years for admission to  Secondary course and 15 years for the Senior Secondary course .

follow the below link to know in details:

https://sdmis.nios.ac.in/