As your domain llompart.com.mx is only using A records and you have defined entries for mail, www, smtp-relay, etc separately you will be safe to just modify the IP address that the www A record has to point at the new web server. You will need to ask them for the IP address or hostname of the actual web server that will host your site.
If they give you an IP address than update the www.llompart.com.mx A record to that address.
If they give you a hostname you can put it in as a CNAME record (ie: www CNAME www5.perfectwide.com.) if you wish. The advantage of the CNAME is if they change an IP address or server later, you don't have to update anything.
If you want users to be able to reach your site just via http://llompart.com.mx then you need to also update the A record defined as either llompart.com.mx. A 126.96.36.199 or it may appear as @ A 188.8.131.52.
I am not familiar with NIC.MX so I can't give you details on where to actually make the changes in their site.
For starters, lets say your domain is xyz.com and these are your current records:
@ IN A 184.108.40.206
server1 IN A 220.127.116.11
www IN CNAME server1
mail IN CNAME server1
IN MX 10 server1.xyz.com.
To break this down a bit:
@ means the origin or base level of your domain, so that line says resolve xyz.com to 18.104.22.168
The server1 line is similar to the @ line, this time saying resolve server1.xyz.com to 22.214.171.124
CNAMEs are basically aliases, instead of resolving to an IP address, they point to another name.
the www line says to resolve www.xyz.com to server1.xyz.com which in turn goes to 126.96.36.199
The 10 in the MX line is the priority, this only really matters if you have multiple MX entries.
If your layout looks similar to this one, your best bet is to change the www record from a CNAME to an A record and point the www and @ to the third party server such as:
@ IN A 188.8.131.52
www IN A 184.108.40.206
Leaving the server1, mail and MX records to remain pointed at your current server.
Your registrar panel will display the entries a little different (won't have the IN for instance) but it should identify A records, CNAMEs and MX entries.
When typing in the destination addresses for the CNAME and MX entries, it is important to pay attention to the ending --> . <-- (I am trying to point out the period here).
When specifying DNS addresses, if you end with a period, that means you have typed the full address out (like my example of server1.xyz.com.) and so that will be the result given. If you do not have the ending period such as (server1) then the domain will be appened to the end (becoming server1.xyz.com).
If you can post details about your A, CNAME and MX records I can update this answer specific to your DNS layout.