You would first need to obtain the id of the Stored Password for which you would like to update.
You can send the following request via a rest client such as postman to obtain that:
HTTP Method: Get
https://GatewayHostName:8443/restman/1.0/passwords?name=MyStoredPasswordName
You will see the following within the generated response:
<l7:StoredPassword id="9d2a35cc52c836e10f76e2ac10eeeeee" version="1">
You can now send the following kind of request to update that stored password:
HTTP Method: Put
https://GatewayHostName:8443/restman/1.0/passwords/9d2a35cc52c836e10f76e2ac10eeeeee
The body of the request would contain something along the following:
<l7:StoredPassword xmlns:l7="http://ns.l7tech.com/2010/04/gateway-management">
<l7:Name>MyStoredPasswordName</l7:Name>
<l7:Password>MyUpdatedPasswordHere</l7:Password>
<l7:Properties>
<l7:Property key="description">
<l7:StringValue></l7:StringValue>
</l7:Property>
<l7:Property key="lastUpdated">
<l7:DateValue>2020-03-19T13:03:37.808-04:00</l7:DateValue>
</l7:Property>
<l7:Property key="type">
<l7:StringValue>Password</l7:StringValue>
</l7:Property>
<l7:Property key="usageFromVariable">
<l7:BooleanValue>true</l7:BooleanValue>
</l7:Property>
</l7:Properties>
</l7:StoredPassword>
The stored password would be updated with the value specified in the <l7:Password>xxxxxxx</l7:Password>
section of the PUT request from above.