These steps assume that you have already added the Team Members either by following the steps in Adding a Team Member to Multiple Records or Add or remove Account Teams using Data Loader or How can Opportunity Teams be added / removed using the Data Loader?
When importing Account Team Members / Opportunity Team Members with the Data Loader it is only possible to specify the role of each user. Access levels default to the least permissive access of Read Only for the Opportunity upon creating Opportunity Team Members and Read Only for the Account and Private for its related objects (Contact, Opportunity, Case) upon creating Account Team Members however, there are some exceptions:
1. If the member being added is a System Administrator the access level will be Read/Write.
2. If the user or group (UserOrGroupId) being added as a Team Member already has sharing established for the related object's then those access levels are used or inherited by the Account Team Member.
To mass update the access level of these members you can use Data Loader's Insert function on the Opportunity Share object or the Account Share object. To see these in the list of available objects in Data Loader select the "Show all Salesforce objects" option.
The fields that you need for Opportunity Share object is to map the Opportunity ID, UserOrGroupID, and OpportunityAccessLevel. The updates will then appear on the Sales Teams related list.
For the Account Share Object you will need to prep your CSV file with the following columns (fields): AccountId, UserOrGroupId, AccountAccessLevel, OpportunityAccessLevel, CaseAccessLevel, ContactAccessLevel.
Note: When preparing your CSV, the AccessLevel(s) field value(s) should be "Read" for Read Only and "Edit" for Read/Write access. If you need to remove granted access for an Account's related object, use the value "None" in your file to set access to Private.
If you encounter an "INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY: insufficient access rights on cross-reference id" message upon inserting AccountShare or OpportunityShare rows check to ensure that the user set in your file does not own the record that you're attempting establish the share for. Although you can create OpportunityTeamMember or AccountTeamMember records for the record owner, it's not possible to insert AccountShare or OpportunityShare records for the current owner of the record.
How to Mass Update Account Teams related objects access