sorry.. DON’T CLEAR THE WP_TERMS TABLE. Just edit those you want to change. Might also require changes in WP_TERMS_TAXONOMY and WP_TERMS_RELATIONSHIP. Check thoroughly and understand the system before doing any editing.
1. Check myPHPadmin
2. Look for wp_terms table
3. Delete the categories you don’t use anymore or just delete everything in that table.
4. Edit your categories in the WP-admin.