Be very careful when using sed to edit XML files. (It's risky).
But you can easily use an XSLT-1.0 processor like xsltproc or Saxon to remove the leading +1 string from the <address> element. So use the following XSLT file
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" />
<xsl:output method="xml" indent="yes" />
<!-- Identity template -->
<xsl:template match="node()|@*">
<xsl:copy>
<xsl:apply-templates select="node()|@*" />
</xsl:copy>
</xsl:template>
<xsl:template match="sms/address[starts-with(.,'+1')]">
<xsl:copy>
<xsl:value-of select="substring(.,3)"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
with your XML and the result (with the XML from your question) is:
<root>
<sms>
<address>5629876543</address>
<date>1554966601000</date>
<type>1</type>
<body> Yea, should be true. </body>
<mmsReaded>1</mmsReaded>
<attachments/>
</sms>
<sms>
<isMms>1</isMms>
<date>1554968044000</date>
<type>2</type>
<mmsMsgBox>2</mmsMsgBox>
<mmsReaded>1</mmsReaded>
<attachments>
<attachment>
<type>image/jpeg</type>
<body/>
<name>Screenshot_20190411-002704_Flud.jpg</name>
</attachment>
</attachments>
</sms>
<sms>
<isMms>0</isMms>
<address>5621234567</address>
<date>1554968778000</date>
<type>1</type>
<isMms>0</isMms>
<address>7141234534</address>
<date>1558919932000</date>
<type>1</type>
<body>:)</body>
<mmsReaded>1</mmsReaded>
<attachments/>
</sms>
<sms>
<isMms>0</isMms>
<address>7141234567</address>
<date>1558927846000</date>
<type>1</type>
<body>It's so</body>
<mmsReaded>1</mmsReaded>
<attachments/>
<isMms>0</isMms>
<address>7145757575</address>
<date>1543704644000</date>
<type>1</type>
<body>Hey</body>
<mmsReaded>1</mmsReaded>
<attachments/>
</sms>
<sms>
<isMms>0</isMms>
<date>1543704676000</date>
<type>2</type>
<body>More text</body>
<mmsReaded>1</mmsReaded>
<attachments/>
</sms>
<sms>
<isMms>0</isMms>
<address>7142323232</address>
<date>1543704736000</date>
<type>1</type>
<body>Lol not even</body>
<mmsReaded>1</mmsReaded>
<attachments/>
</sms>
<sms>
<isMms>0</isMms>
<address>7141010101</address>
<date>1543704748000</date>
<type>1</type>
<body>You do</body>
<mmsReaded>1</mmsReaded>
<attachments/>
</sms>
</root>
This should be as desired.