Ok to do this you can follow these simple steps
go inside phpmyadmin and in your database
there in run this sql query in side wp_options table
If you have custom prefix for your wordpress tables then
"custom_prefix_options"
SELECT * FROM `custom_prefix_options` where option_name="template" OR option_name="stylesheet"
Here you need to add theme name in both "option_value" fields
This will select that theme as active theme