Hey,
from what I found here, here and here, it might work to add the following lines to /etc/slapd/slapd.conf :
rootdn "cn=admin,dc=yunohost,dc=org"
rootpw {SSHA}O4kkm2OkgO2DPrrnYXXXXXXXXXXXXXXX
where the hash in the last line comes from (or at least that’s my understanding)
slappasswd -h {SSHA}
# A password will be ask, and you'll get the corresponding hash
Once the lines are added, [here you might need to restart the ldap service ?], you should be able to connect to the admin interface, rechange the password properly, remove the lines you added in slapd.conf, and that should be all.
Hope it helps !