myfirstltng.cmp

<aura:component implements="force:appHostable" controller="myfirstltngController">
    <aura:attribute name="account" type="Account"/>
    <aura:handler name="init" action="{!c.init}" value="{!this}"/>
    {!v.account.Name}
    <button onclick="{!c.save}">click</button>
</aura:component>

myfirstltngController.cls

public with sharing class myfirstltngController {

    @AuraEnabled
    public static Account retrieveAccount() {
        return [select Id, Name from Account limit 1];
    }

    @AuraEnabled
    public static void saveAccount(Account account) {
        system.debug(account);
        upsert account;
    }

}

myfirstltngController.js

({
    init : function(component, event, helper) {
        var action = component.get('c.retrieveAccount');
        action.setCallback(this, function(data) {
            component.set('v.account', data.getReturnValue()); 
        });
        $A.enqueueAction(action);
    },
    save: function(component, event, helper) {
        component.set('v.account.Name', component.get('v.account.Name') + 'x')
        var action = component.get('c.saveAccount');
        action.setParams({account: component.get('v.account')});
        $A.enqueueAction(action);
    }
})