Please add/modify the following line in the  Midtier's config.properties file(one line):

*config.properties*********
arsystem.password=OdMEFDAE14nHXLPK4ugMQ+Hq4SADahRZnybhmXKBowW2PuX23Bbdc46iYqD+WnQSP6czHTS4fd/+d1ascWp3KNoe65QZg51Fd60Nr6rGF0s3dnRw2oBOgw\=\=
*config.properties*********
(usually on .....AR System\Mid-Tier\WEB-INF\classes\config.properties)

Then restart the servlet engine(web server). Now the new password for midtier configuration tool would be "arsystem"

Now, please login into the Mid-tier configuration tool with "arsystem" as password.  Please change it after.

After resetting password, it is recommended to do hard cache flush in order to avoid issues when accessing to http://<MT server>:<port>/arsys

Below is KA how to do hard cache flush :-
AR System Mid Tier - How to flush the Remedy AR System Mid Tier cache - INCLUDES VIDEO